Light-mediated regulation of glutamine synthetase activity in the unicellular cyanobacterium...

Planta (1992)187:247-253 P l a n t a

�9 Springer-Verlag 1992

Light-mediated regulation of glutamine synthetase activity in the unicellular cyanobacterium Synechococcus sp. PCC 6301 Siivia Marqu6s*, Angel M6rida, Pedro Candau, and Francisco Javier Florencio**

Departamento de Bioquimica Vegetal y Biologia Molecular, Instituto de Bioquimica Vegetal y Fotosintesis, Universidad de Sevilla-CSIC. Apartado 1113, E~41080 Sevilla, Spain

Received 30 July; accepted 16 October 1991

Abstract. Glutamine synthetase (GS; EC 6.3.1.2) activity from the unicellular cyanobacterium Synechococcus sp. strain PCC 6301 shows a short-term regulation by light- dark transitions. The enzyme activity declines down to 30% of the original level after 2 h of dark incubation, and can be fully reactivated within 15 min of re-illumination. The loss of activity is not due to protein degradation, but rather to a reversible change of the enzyme, as deduced from the GS-protein levels determined in dark-incubated cells using polyclonal antibodies raised against Synecho- coccus GS. Incubation with 3-(3-4-dichlorophenyl)- 1,1- dimethylurea (DCMU) also provokes GS inactivation, indicating that an active electron flow between both photosystems is necessary to maintain GS in an active state. On the other hand, the light-mediated reactivation of GS in dark-incubated cells treated with dicyclohexylcarbodiimide (DCCD) or carbonyl cyanide m-chlorophenylhydrazone (CCCP) indicates that neither changes in the ATP synthesis nor the lack of an electrochemical proton gradient across the thylakoid membrane are directly involved in the regulation process. The inactive form of GS is extremely labile in vitro after disruption of the cells, and is not reactivated by treatment with dithiothreitol or spinach thioredoxin m. These results, taken together with the fact that dark-promoted GS inactivation is dependent on the growth phase, seem to indicate that GS activity is not regulated by a typical redox process and that some other metabolic signal(s), probably related to the ammonium-assimilation pathway, might be involved in the regulation process. In this regard, our results indicate that glutamine is not a regulatory metabolite of Synechococcus glutamine synthetase.

* Present address: GBF, Abteilung fiir Mikrobiologie. Masche- roderweg 1, W-3300 Braunschweig, Federal Republic of Germany

** To whom correspondence should be addressed Abbreviations: CAP = chloramphenicol; CCCP = carbonyl cyanide m-chlorophenylhydrazone; DCCD = dicyclohexylcarbodiimide; DCMU = 3-(3-4-dichlorophenyl)- 1,1-dimethylurea; DTT = dithiothreitol; GOGAT = glutamate synthase ; GS = glutamine synthetase ; PFD =photon flux density

Key words: Ammonia/ammonium assimilation - Cyano- bacteria - Glutamine synthetase - Photosynthesis (glutamine synthetase activity) - Synechococcus

Introduction

In cyanobacteria, ammonium assimilation takes place through the sequential action of glutamine synthetase (GS; EC 6.3.1.2) and glutamate synthase (GOGAT; EC 1.4.7.1) (Meeks et al. 1978) in a pathway that requires both ATP and reducing power. Glutamine synthetase carries out the ATP-dependent synthesis of glutamine from NH2 and glutamate, and represents the connecting step between carbon and nitrogen metabolism.

The enzyme from cyanobacteria has been purified and well characterized in both dinitrogen-fixing and non- fixing organisms (Orr et al. 1981 ; Florencio and Ramos 1985), and is composed in all cases of 12 identical sub- units of about 50 kDa arranged in two superimposed hexagonal rings, resembling very closely the enzyme from enterobacteria (Orr et al. 1981 ; M6rida et al. 1990).

In cyanobacteria, it is well established that the assimilation of inorganic nitrogenous compounds is a genuine photosynthetic process (Guerrero and Lara 1987), thus liable to be light-regulated, as happens with several enzymes involved in CO2 fixation. In Synechococcus sp. PCC 6301, several enzymes involved in photosynthetic carbon metabolism have been shown to be light-modulated (Duggan and Anderson 1975); in all cases, light acted as an activator, the only exception being glucose-6- phosphate dehydrogenase which is inactivated in the light. Little is known about the mechanisms involved in the light modulation of the enzymes in cyanobacteria. However, in-vitro studies seem to indicate that the same ferredoxin/thioredoxin system described for higher plants could also be operative in this group of organisms (Crawford et al. 1984).

A light-dependent regulation of GS activity has been found in higher plants (Taylor and Stewart 1980), green

248 S. Marqu6s et al.: Light regulation of cyanobacterial glutamine synthetase

algae (Tischner and H i i t e r m a n n 1980), photosynthe t ic bacter ia (Nord lung et al. 1985) and cyanobac te r ia (Row- ell et al. 1979), a l though very different regulatory mechanisms seem to be involved in each case.

The regula t ion of cyanobacter ia l GS activity is no t clear, a l though amino acids (acting in a cumula t ive man- ner), d ivalent cat ions, and thiols have been proposed as modu la to r s of the activity (Ip et al. 1983; Papen and Bothe 1984). There is no evidence of covalent modifica- t ions, such as adenyly la t ion , o f a GS f rom any cyanobacteria (Fisher et al. 1981).

In this paper , we describe and characterize a reversible shor t - te rm regula t ion of GS by l ight-dark t rans i t ions in Synechococcus sp. PCC 6301 that is coincident with changes in the reducing condi t ions of the cells and the levels of metabol i tes related to the G S - G O G A T pathway.

Material and methods

Organism and culture conditions. Synechococcus sp. PCC 6301 from the Sammlung von Algenkulturen am Pflanzenphysiologischen In- stitut der Universit~it, G6ttingen, FRG was grown in 1-1 Roux flasks or 100-ml tubes under continuous illumination (125 ~tmol photons �9 m- 2. s i, white light from fluorescent lamps) at 40 ~ C on a synthetic medium (Allen and Arnon 1955) supplemented with 20 mM potassium nitrate, and bubbled with a stream of 1.5% (v/v) CO z in air. Dark conditions were obtained by wrapping the flasks in aluminium foil.

Enzyme assays. Glutamine synthetase biosynthetic (Marqu6s et al. 1989) and transferase activities (Shapiro and Stadtman 1970) were determined as previously described. For the in-situ assays of GS activities, a sample of Synechococcus cells containing about 25 mg of chlorophyll (chl) was permeabilized in 1 ml of the corresponding reaction mixture by the addition of 20 ml of 1.25% (w/v) mixed alkyltrimethylammonium bromide, and the enzyme activity was then determined as previously described (Marqu6s et al. 1989). One unit (U) of GS biosynthetic activity was the amount of enzyme that produced 1 lamol of glutamine per minute. One U of GS transferase activity was the amount of enzyme that produced 1 lamol of "/-glutamylhydroxamate per minute.

Analytical methods. Determination of intracellular amino acids was carried out in cell lysates obtained by adding aliquots of cell suspen- sions equivalent to 0.4 mg protein to ice-cold 0.2 N HCI, followed by centrifugation (12000 "9, 20 min at 4 ~ C). The amino-acid concentration in the samples was determined by high-performance liquid chromatography (HPLC) as described in (Marqu6s et al. 1989).

For the determination of the intracellular 2-oxoglutarate concentration, cells from 80 ml of a Synechococcus culture were harvested by centrifugation at 5000 �9 g for 10 min at 4 ~ C. The pellet was resuspended in 1 ml (final volume) of cold 0.3 M HC104 and incubated for 15 min at 4 ~ C. The cell lysates obtained were centrifuged at 12000-9 for 20 rain at 4 ~ C, and 2-oxoglutarate in the supernatant was analyzed using commercial glutamate dehydrogenase as described by Senior (1975).

Protein content was determined by the method of Lowry modi- fied as previously described (Markwell et al. 1978) for whole cells, or by the method of Bradford (1976) for cell-free extracts using ovalbumin as standard. Chlorophyll content was determined by the method of Mackinney (1941).

Crude-extract preparation. Cells (250 ml) were harvested by centrifugation at 5000 �9 g at 4 ~ C for 15 min and resuspended in 4.5 ml of

50 mM potassium-phosphate buffer (pH 7.5), supplemented with 1 mM phenylmethylsulfonyl fluoride (PMSF). The suspension was disrupted by sonication in a 0 ~ C waterbath for 5 min in 30-s bursts (100 W, 20 kHz), and centrifuged at 18000 g at 4 ~ C for 15 min. The pellet (membrane-bound fraction) was then resuspended in 0.5 ml of the same buffer.

Wavelength-dependent reactivation. Synechococcus cells were incubated in darkness for 3 h and then a 3-ml aliquot was loaded into a 1-cm-diameter cylindrical cuvette which was open to the air. The cells were incubated under monochromatic light with continuous stirring at 35 ~ C for 10 min. The light source was a 200-W Mercury- Xenon arc lamp (Hanovia 901-BI ; Oriel Corp., Stratford, Conn., USA) modulated at a current of 10 A with a 8500 Oriel universal power supply. Exciting monochromatic light was obtained by using bandpass optical filters (Baird-atomic) with central wavelengths of: 425, 480, 500, 555, 600, 650 and 720 nm. Incident photon flux density (PFD) on the surface of the cuvette was measured with a Li-188 integrating quantum radiometer/photometer provided with a Li 190 SB quantum-sensor cell (Li-Cor).

Immunological procedures. Polyclonal monospecific antibodies against Synechococcus sp. PCC 6301 GS were raised by injecting rabbits with homogeneous GS enzyme (Florencio and Ramos 1985). Immunoelectrophoresis was performed by the method of Laurell and Mckay (1981) except that 100 mM glycine, 37.5 mM Tris, 1% (v/v) Triton X-100, pH 8.6, was used and that 75 ml of GS-specific antiserum was added to 18 ml of buffered 1% agarose solution. Samples were incubated for 20 min at 4 ~ C in the presence of 1% (v/v) Triton X-100, and a 10-1al aliquot was applied to each well. Electrophoresis was carried out at 10 V �9 cm 1 in a (Phar- macia, Uppsala, Sweden) flat-bed electrophoresis chamber for 16 h at 10 ~ C. Finally the gel was repeatedly washed with 0.15 M NaCI, dried with filter paper and stained for protein.

Results and discussion

Inactivation o f GS in darkness. Synechococcus cells growing at sa turat ing P F D and using ni trate as the only n i t rogen source had high levels of GS activity (1.5 U - (mg protein)-1, transferase activity, or 90 m U �9 (mg protein)-1, biosynthet ic activity) that were ma in t a ined dur ing the first growth phase of a no rma l culture. W h e n cells growing under these condi t ions were t ransferred to darkness, a progressive decline in GS activity was observed, reaching values of 30 % of the original levels after 2 h of incuba t ion (Fig. 1). The decay was similar for bo th biosynthet ic and transferase activities, conf i rming the physiological significance of this inact ivat ion. In early studies, Rowell et al. (1979) described a similar dark- inact iva t ion of GS activity in several species of cyanobacteria, inc luding Syneehococeus sp. PCC 6301. However, the inact iva t ion process was much slower and the final inac t iva t ion obta ined after 12 h of dark incuba t ion never reached 50% of the init ial values, in cont ras t to our results where more than 80 % inact iva t ion was observed in a much shorter t ime (2-3 h). This discrepancy may result f rom the fact that these authors assayed the enzyme activity in vitro after prepar ing crude extracts of the cells, while we did the assays in situ in permeabi l ized cells. We have observed that total d is rupt ion of the cells p roduced a substant ia l react ivat ion of the enzyme (see below).

Da rk inac t iva t ion of GS can be prevented by chloramphenicol (CAP). This fact could indicate the

S. Marqu6s et al. : Light regulation of cyanobacterial glutamine synthetase 249

120

100

transferase

80

:~ ~ O 60 ~ e t i c

if) 40

20 Dark

0 �9 , . . . . . i , i . , . , . , . , .

0 40 80 120 160 200

Time (min) Fig, 1. Time-course of dark inactivation of the GS activity. A culture of Synechococcus sp. ceils (9.9 gg Chl �9 m l - 1) growing in the light with nitrate as nitrogen source was divided at zero time into two halves. One half was kept in the light (open symbols) and the other was transferred to darkness (filled symbols). Glutamine synthetase biosynthetic and transferase activities were determined as described in Material and methods. The 100% GS-activity values were 2.38 U - ( m g Chl) -1 (biosynthetic) and 46 U . ( m g Chl) -1 (transferase)

120

100~

80

�9 ~- 60

t/~ 40 (5

�9 Dark+ CAP(0min) 20 [] Dark+ CAP (30 rnin)

& Dark+ CAP (60 rain) 0 I I ! �9 ! �9 ! �9 ! q !

0 20 40 60 80 100 120 140 160

T i m e (rain) Fig. 2. Effect of the addition of CAP during the dark inactivation of GS. A culture of Synechococcus cells growing in the light (9.67 gg Chl �9 m l - 1) was divided at zero time into five fractions that were incubated under the following conditions: light, darkness, and darkness plus CAP (75 pg/ml) added at 0, 30 and 60 rain after incubation. The GS transferase activity was determined in situ as described in Material and methods. The 100% GS-activity value corresponds to 62.2 U �9 (mg Chl)-1

possible involvement of protein synthesis in the inactivation process. However, GS inactivation was immediately stopped when CAP was added after the process had already begun (Fig. 2), ruling out the need for an inac- tivating protein and pointing to some other alteration of the cell caused by CAP treatment, such as a change in the pools of amino acids as a consequence of the inhibition of protein synthesis.

The degree of inactivation of GS in darkness appeared to be dependent on the culture age and-or density. There

TaMe 1. Glutamine synthetase activity and protein in crude extracts from dark- or light-incubated cells. A culture of Synechococcus cells growing in the light (12.2 I~g Chl . m1-1) was divided into two 300-ml fractions. One of them was maintained in the same conditions and the other transferred to darkness. After 5 h, GS transferase activity was determined in situ: 37 U (mg �9 Chl)- 1 (light) and 2 U (mg. Chl)- 1 (darkness). Samples of 250 ml of each fraction were harvested and extracts were prepared as described in Material and methods. Glutamine synthetase transferase activity and GS protein were determined in vitro in bo th the supernatant and the pellet fractions of a 18 000 �9 g centrifugation of the crude extract

Fraction Glutamine synthetase

Protein (relative area) (arbitrary units)

Specific activity (mU �9 (mg protein)-1)

Light Darkness Light Darkness

Supernatant 2.33___0.35 2.33+_0.32 74.2+_4,0 20.9+_1.5 Pellet 0.60+0.05 0.32-t-0.03 118.9___7,0 27.6_+2.2

was an optimal stage in the growth curve, corresponding to the linear phase, where the susceptibility of the GS to dark switch-off was maximal. The inactivation was lower or totally absent before and after this period. A dependance on growth phase has also been observed in the photosynthetic bacterium Rhodospirillum rubrum where both dark- and ammonium-promoted switch-off of its nitrogenase correlated with the variation of internal metabolic pools throughout the growth phase (Li et al. 1987).

In order to determine whether the decrease in GS activity was due to protein degradation or to inactivation of the enzyme, the amounts of GS protein present in the cells in the light and after the dark switch-off were measured by immunoelectrophoresis, using specific antibodies raised against Synechococcus sp. PCC 6301 GS. During the preparation of crude extracts GS protein was recovered from both the supernatant and the membrane- bound fraction (pellet). Table 1 clearly shows that although the GS activity was much higher in extracts from cells cultivated in the light than in cells transferred to darkness for 5 h, the same amount of GS protein was found in both cases. We can therefore conclude that the decay in GS activity in the dark is a post-translational inactivation process that does not imply any detectable enzyme degradation. The rapid reactivation of the enzyme that occurred upon illumination of cells containing dark-inactivated enzyme confirms this conclusion (see below). The area in the rocket immunoelectrophoresis corresponding to the GS present in the pellet fractions is probably an underestimation of the real amount of GS present, owing to an incomplete sol- ubilization of the membrane-bound protein in these fractions (Florencio and Ramos 1985). This could explain the difference that exists between supernatant and pellet in the ratios of activity/relative area (see Table 1).

Light reactivation of GS. Effect of light quality on the reactivation. When Synechococcus cells with low levels of GS activity after a sustained incubation in the dark (Fig. 1) were returned to standard conditions of illumination,

250 S. Marqurs et al. : Light regulation of cyanobacterial glutamine synthetase

120

100 gmo l pho tons r n ' 2s -1 25 gmo l pho tons m-2s -1

lOO

2 ~tmol photons m'2s "1

v i ~ I I I I 0

0 5 10 15 20

T i m e ( ra in)

Fig. 3. Effect of light on GS reactivation. A culture of light-grown Synechococcus cells (15 gg Chl, ml-1, GS transferase activity = 47.2 U �9 (rag Chl)- 1) was transferred to darkness for 3 h. At zero time, fractions from this culture were illuminated at the indicated PFDs and GS transferase activity was determined

60

40

20

the original high levels o f activity were rapidly regained (Fig. 3). The average half-life o f the process was less than 10 min, with 85% of the total activity recovered after 15 rain. Again, both transferase and biosynthetic activities reached their original values with the same kinetics (data not shown). Figure 3 also shows the effect of different PFDs of white light on the reactivation kinetics of the GS of Synechocoecus inactivated after several hours of dark incubation. I t is worth noting that very low PFDs are sufficient to obtain a partial reactivation of the enzyme. The time required for full reactivation could be shortened to 5 min when the P F D was increased to 250 lamol �9 m - 2 . s-~ (data not shown). As was expected, the reactivation process was independent of protein synthesis. The same rate and degree o f reactivation occurred in the presence of either CAP (70 lag- m l - i) or rifampicin (20 lag �9 ml-1) as with no addition, confirming the posttranslational nature of the inactivation process (data not shown).

The effect o f light o f different wavelengths on the l ight-mediated reactivation of GS is shown in Fig. 4. Maximal efficiency o f reactivation was achieved at wavelengths ranging f rom 600 to 680 nm, associated with the main absorpt ion band of PSII (Chapman 1973), thus indicating that the bulk of the excitation energy which drives GS reactivation is provided by this photosystem. Even if the part icipation of PSI cannot be completely excluded, the prevention of GS activation by 3-(3-4-di- chlorophenyt)- l , l -d imethylurea ( D C M U ) indicates that PSI does not play a critical role in the reactivation process. The effects of the different wavelengths shown in Fig. 4 were only obtained at very low PFDs (8-10 l a m o l . m - Z - s - 1 ) since PFDs higher than 100 lamot, m -2 - s -1 produced a full reactivation of the enzyme at any wavelength used, probably because of

11o e- .9 1oo

9o

a0

7o

i~ 6o o

5o g) 1.9 40

400 450 500 550 600 650 700 750

W a v e l e n g t h (nm)

Fig. 4. Effect of different light wavelengths on GS reactivation. Synechocoecus cells (15 gg Chl- ml- i) maintained in darkness (3 h) were illuminated with light of different wavelengths (PFD from 8 to 10 gmol. m-2. s- 1). After 10 min of illumination GS transferase activity was determined. The 100% GS-activity value was 47.2 U " (mg Chl)-1

v

(t) r

120 Light + DCMU + CAP

100'

8O

60

40

2O

0 = i . i . i . i . I . i . i . ! �9 i ,

40 80 120 160 200

T i m e ( r a i n )

Fig. 5. Effect of DCMU on GS activity A culture of Synechococcus cells growing in the light (13.6 gg Chl- ml-a) was divided at zero time into four fractions that were incubated under the following conditions: light plus 20 gM DCMU; darkness; light plus 20 gM DCMU plus CAP (75 gg �9 ml- I), and a control consisting of light with no addition, whose GS transferase activity was taken as 100%. The GS transferase activity was determined in situ as described in Material and methods. The GS activity at zero time was 46.2 U - (rag Chl)- 1

contaminat ion by low-intensity light o f other wavelengths. This fact, taken together with the results shown in Fig. 3, indicates that the electron flow needed for full light reactivation of the enzyme is very low.

Effect of photosynthetic inhibitors on the regulation of GS. The most obvious consequence of dark t reatment of cyanobacteria is the cessation of photosynthetic electron flow. We have tried to simulate dark conditions using several compounds that interact with the photosynthetic process. The addition of D C M U to cells growing in t h e

light produced a decline in GS activity similar to that obtained in darkness. This inactivation was again prevented by the presence of CAP (Fig. 5). The benzo-

S. Marqu6s et al. : Light regulation of cyanobacterial glutamine synthetase 251

120

100

80

60

o CONItK)k

.

40

20

0 T , �9 0 ~ �9 i , i , | , i , i |

0 10 20 30 40 50 60 70

T ime (rain)

Fig. 6. Effect of photosynthesis inhibitors on the light-reactivation of GS inactivated by darkness. A culture of Synechococcus cells growing in the light was transferred to darkness for 3 h and was then divided into four fractions that were exposed to the light (125 lamol photons �9 m -2 �9 s - t ) under the following conditions: control with no addition; plus 10 gM CCCP; plus 20 laM DCCD; plus 20 gM DCMU; and plus CAP (75 ~tg - ml-1). The GS transferase activity was determined in situ as described in Material and methods

quinone-ferricyanide system, known to interrupt the electron flow to PSI at the quinone level in vitro (Trebst 1972), had in vivo a similar effect to DCMU or darkness on GS activity (data not shown). On the other hand, when the re-illumination of the dark-incubated cells was carried out in the presence of 20 gM DCMU, the reactivation of GS was totally prevented (Fig. 6). These results indicate that the signal responsible for the inactivation of GS in the dark requires a cessation of the electron flow between the two photosystems, and that restoration of the electron flow is needed for the reactivation of the enzyme.

Glutamine synthetase activity of light-grown cells was not affected by either dicyclohexylcarbodiimide (DCCD), an irreversible inhibitor of ATPases (Solioz 1984), or carbonyl cyanide m-chlorophenylhydrazone (CCCP), a photosynthetic-flow uncoupler that inhibits ATP synthesis in cyanobacteria (Bottomley and Stewart 1976). On the other hand, the presence of CCCP slightly (but consistently) accelerated the reactivation of the enzyme, probably as a consequence of an increase in the production of reductants in uncoupled cells. Although unable to inhibit the rapid initial reactivation, DCCD prevented it from reaching its maximal value (Fig. 6). These results strongly indicate that neither changes in ATP synthesis nor the lack of electrochemical proton gradient across the membrane are directly involved in the inactivation process, and that the reactivation is also independent of ATP synthesis or H § gradient. The lesser effect on reactivation observed with DCCD could be the consequence of a blockage in the normal electron flow due to inhibition of ATP synthesis. All these data indicate that a functional PSII is required to maintain the GS of Synechococcus in a fully active state.

In-vitro studies with the dark-inactivated GS. A characteristic of the inactivation of GS by darkness is its labil- ity. When extracts were prepared from dark-incubated cells with very low activity levels, as determined by in-situ assay, a significant reactivation of the enzyme occurred during the process. For instance, in the experiment described in Table 1, the levels of activity in extracts from dark-treated cells were about 25% of the level present in extracts from cells maintained in the light, although before the disruption of the cells the activity determined in situ in the dark was only 5% of that in the light. This fact seems to indicate that the inactivation suffered by the enzyme in vivo is highly unstable, once the cells have been disrupted.

Several enzymes have been reported to be light- regulated in photosynthetic organisms. Two characteris- tics are common to all the mechanisms described: (i) the light effect requires a functional electron-transport system; (ii) in vitro, light can be replaced by an artificial reductant such as dithiothreitol (DTT) and thioredoxin. Although the regulation process described above has the first characteristic, in our hands no reactivation was obtained when extracts containing dark-inactivated GS were incubated with either DTT, or spinach thioredoxin rn reduced by DTT. These results differ from those reported for the GS enzyme from the green alga Chlorella, that has been shown to be reactivated by thioredoxin (Tischner and Hfitermann 1980), or for that from the filamentous cyanobacterium Anabaena cylindrica, whose activity could be recovered in vitro by adding 50 mM 13-mercaptoethanol to crude extracts (Rowell et al. 1979). The recent finding in Synechococcus sp. PCC 6301 (Har- rison et al. 1990; Tsinoremas et al. 1991) of a PH-like protein - the regulatory protein that in Escherichia coli controls the adenylyl-transferase activity responsible for the modulation of GS activity (Son and Rhee 1987) - that is regulated by phosphorylation according to the light quality and the nitrogen source, opens the question of the existence of a mechanism of regulation similar to the one functioning in enter�9 in response to nitrogen availability (Shapiro and Stadtman 1970). How- ever, our attempts to reactivate the enzyme by deadeny- lylation or dephosphorylation, incubating under suitable conditions extracts containing inactive enzyme with phosphodiesterase or alkaline phosphatase, respectively, have been unsuccessful (data not shown).

Changes in amino-acid and 2-oxoglutarate pools during light-dark transitions. We have determined, in both light and darkness, the intracellular levels of several amino acids, especially those related to the GS-GOGAT pathway, and of 2-oxoglutarate, which is the carbon skeleton for this route. Table 2 shows the levels of these metabolites in Synechoeoeeus cells subjected to different treatments. It can be seen that these levels declined from 50 to 80% after 2 h of dark incubation, and that the same occurred when DCMU was added. When dark- incubated cells were re-illuminated, an increase in the intracellular levels of these compounds and a rise in GS activity were found. In addition, cells incubated in darkness in the presence of CAP, which prevents GS inactiva-

252 S. Marqu6s et al. : Light regulation of cyanobacterial glutamine synthetase

Table 2. Levels of amino acids, 2-oxoglutarate and GS activity of Synechococcus cells. Metabolites and GS transferase activity were determined as described in Material and methods. The treatments were as follows: none: light-grown cells; darkness: cells transferred for 3 h to the dark; reillumination: cells incubated for 3 h in the

dark and then reilluminated under standard conditions for 2 h; light+DCMU: cells incubated for 3 h in the light plus DCMU (20 ~tM); darkness+CAP: cells incubated for 3 h in the dark plus CAP (75 gg �9 ml 1). Asp, aspartic acid; Glu, glutamic acid; Gln, glutamine; ~OG, 2-oxoglutarate

Treatment Metabolites (nmol - (mg protein)- 1)

Glu Gin Asp 2-OG

GS activity (U �9 (mg protein)-1)

None 39+_3.5 2.8+_0.30 5.1 _+0.50 1.10 +- 0.10 1.61 +-0.08 Darkness 22 +- 2.0 1.7 +- 0.20 1.7_+ 0.18 0.23 _+ 0.25 0.44 + 0.02 Reillumination 35 _+ 3.2 2.1 +- 0.25 4.2 +- 0.38 0.98 _+ 0.09 1.27 _+ 0.04 Light § DCMU 20+ 1.8 1.5+-0.13 2.2+_0.23 0.40+-0.03 0.16+-0.01 Darkness + CAP 32 +- 2.7 2.4 _+ 0.27 4.1 +_ 0.36 0.73 +_ 0.05 1.15 +_ 0.04

tion, showed levels of these metabolites similar to those found in control (not dark-treated) cells. In all cases, there was a clear correlation between the levels of these compounds and GS activity.

It has been previously reported that addition of ammonium to Synechococcus cells grown on nitrate also promotes changes in these amino-acid pools, as well as a partial inactivation of GS (Mrrida 1990), and that the same occurs in other unicellular cyanobacteria such as Synechocystis sp. PCC 6803 (Mrrida et al. 1990, 1991). In the dinitrogen-fixing filamentous cyanobacterium Anabaena cylindrica it has been shown that glutamate levels decline after transition to darkness, together with a decay in GS activity (Rowell et al. 1979), although the time-scale was longer than in our case (8 versus 2 h). In that case a redox regulation of the GS enzyme was proposed (Rowell et al. 1979). It is also worth noting that purified GS enzymes from Synechococcus and Anabaena have been reported to be strongly inhibited by alanine and glycine in vitro (Orr and Haselkorn 1981; Florencio and Ramos 1985), but that seems not to be the case in the change in GS activity studied here, since dark-treated GS remained inactive in the in-situ assay, despite the huge dilution of the intracellular metabolites caused by rendering the cells permeable.

The dependance of the GS inactivation on the growth phase of the culture, together with the parallel behavior of metabolite pools and GS activity under all the conditions studied, seem to indicate that GS activity in Synechococcus cells could be modulated by the internal balance among some metabolites, in a similar way to that described for enterobacteria, where GS activity is con- trolled by the internal ratio between glutamine and 2-oxoglutarate (Shapiro and Stadtman 1970; Senior 1975). This inactivation might be promoted by a change in the available nitrogen source (from nitrate to ammonium) or by the cessation of CO2 fixation in dark-incubated cells. Although we have not yet identified the compound(s) that could modulate GS activity in Synechococcus cells, the data in Table 2 show that the level of glutamine probably does not change sufficiently from light to darkness to merit glutamine alone being considered as a regulatory metabolite of GS in this cyanobacterium.

This work has been financed by the Direcci6n General de Inves- tigaci6n Cientifica y Trcnica, (Grant PB88-0020) and by the Junta de Andalucia, Spain.

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