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APOPTOSIS –THE PROGRAMMED CELL

DEATH

SUBMITTED TO- Dr. KAMLA CHAUDHARY

PRESENTED BY- SHALINI SAINIM.Sc- 3RD SEM MOLECULAR MICROBIOLOGY

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INTRODUCTION• In eukaryotes, the classical form of programmed cell

death (PCD) is apoptosis, which has as its specific characteristics DNA

• Fragmentation and membrane depolarization. In Escherichia coli a different PCD system has been reported.

• It is mediated bythe toxin–antitoxin system module mazEF. The E. coli mazEF module is one of the most thoroughly studied toxin–antitoxin systems.

• MazF encodes a stable toxin, MazF, and mazE encodes a labile antitoxin, MazE, which prevents the lethal effect of MazF.

• mazEF-mediated cell death is a population phenomenon requiring the quorum-sensing pentapeptide NNWNN designated Extracellular Death Factor (EDF).

• mazEF is triggered by several stressful conditions, including severe damage to the DNA.

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• Under the conditions of severe DNA damage, the triggered mazEF-mediated cell death pathway leads to the inhibition of a second cell death pathway.

• The latter is an apoptotic-like death (ALD); ALD is mediated by recA and lexA. The mazEF-mediated pathway reduces recA mRNA levels.

• Based on these results, we offer a molecular model for the maintenance of an altruistic characteristic in cell populations.

• The ALD pathway is inhibited by the altruistic EDF-mazEF-mediated death pathway.

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INTRODUCTION

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Apoptosis pathway in E.coli• MazEF system of E. coli. The mazEF system of E.

coli is one of the best studied. It was first described as a mechanism responsible for bacterial PCD .

• The mazF gene encodes a stable cytotoxic protein, and mazE encodes an unstable antidote to MazF protein, which is quickly degraded by the ATP-dependent ClpAP serine protease [ Under balanced growth conditions, as long as MazE and MazF are coexpressed, MazE neutralizes the toxic effect of MazF i.e. the toxin is constantly neutralized by the antitoxin. The MazF toxin is a “long-lived” protein, an endoribonuclease, which cleaves RNA in ACA sequences .

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• However, under stress conditions (starvation, effects of antibiotics, reactive oxygen species, presence of 3,5′-bispyrophosphate (ppGpp), DNA damage), which prevent expression of the mazEF operon, the equilibrium is disturbed, and the toxin “poisons” the cell, destroying the cellular mRNA, which leads to cell death and autolysis of the greater part of the population

• Under stressful conditions, the labile protein antitoxin MazE is destroyed by the ClpAP protease, and as a result the more stable MazF toxin is free to act, ultimately causing cell death.

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•  Extensive DNA damage can lead to cell death following a second pathway called apoptotic-like death (ALD)

• . ALD is similar to apoptosis of eukaryotic cells. The cell membrane is depolarized and DNA is fragmented in the course of this process. ALD to result from the activity of two proteins, RecA and LexA.

• . LexA protein is an inhibitor of the SOS response, which is a global response of a bacterial cell to DNA damage, when the cell cycle is interrupted and DNA reparation is induced. Thus, ALD can be defined as a form of SOS response.

• In E. coli cells, the ALD pathway is a backup system in relation to mazEF, the main pathway of cell death . If some components of the mazEF pathway are inactivated, bacterial cell death will follow the ALD pathway

• . In this case RecA inactivates LexA, which is a repressor of the transcription of SOS-system genes. Essentially, the destruction of LexA protein activates the SOS response. The mazEF system-mediated mechanism of cell death also inhibits the ALD pathway by reducing the mRNA level of RecA protein.

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CENTRAL REGULATOR

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THE FUNCTION OF ROS AND YihE

•  The YihE protein kinase is a key regulator that protects Escherichia coli from antimicrobial and environmental stressors by antagonizing the MazEF toxin-antitoxin module.

• YihE was linked to a reactive oxygen species (ROS) cascade, and a deficiency of yihE stimulated stress-induced PCD even after stress dissipated.

• YihE was partially regulated by the Cpx envelope stress-response system, which, along with MazF toxin and superoxide, has both protective and destructive roles that help bacteria make a live-or-die decision in response to stress.

• YihE probably acts early in the stress response to limit self-sustaining ROS production and PCD. Inhibition of YihE may provide a way of enhancing antimicrobial lethality and attenuating virulence.

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The enteric bacterium E.coli bacteria, carries on its chromosome a pair of genes, mazEand mazF (mazEF): mazF specifies a toxin, and mazEspecifies an antitoxin. Previously, we have shown that E.coli mazEF is responsible for bacterial programmed cell death in response to stressors such as DNA damage. Here, the extensive DNA damage can induce a second mode of cell death, which we call apoptotic-like death (ALD). ALD is like apoptosis—a mode of cell deaththat has previously been recorded only in eukaryotes.During ALD, the cell membrane is depolarized, and theDNA is fragmented and can be detected using the classicalTUNEL assay.

THE PCD PATHWAY IN E.coli

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The MazEF death pathway, however, shows neither of those features, yet also kills the cell. Here shown that ALD is mediated by two proteins, RecA and LexA, which are noteworthy because LexA is an inhibitor of the SOS response (which is a global response to DNA damage in which the cell cycle is arrested and DNA repair isinduced). This defines ALD as a form of SOS response.Furthermore, MazEF and its downstream componentscause reduction of recA mRNA levels, which could explainhow the MazEF pathway inhibits the ALD pathway .of the components of the mazEF pathway be inactivated,bacterial cell death would occur through ALD.

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The E. coli ALD pathway is a back-up system for the traditional mazEF cell death pathway. Should oneof the components of the mazEF pathway be inactivated,bacterial cell death would occur through ALD. These findings also have implications for the mechanisms of‘‘altruistic’’ cell death.

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COMMON FUNCTION IN ALL BACTERIAL CELL

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The genes switched on and off during apoptosis

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References• Microbiology and molecular biology riveiws10.1128/MMBR.64.3.503-

514.2000. Microbiol. Mol. Biol. Rev. 2000, 64(3):503. DOI: Kim Lewis Programmed Death in Bacteria.

Two Programmed Cell Death Systems in Escherichia coli: An Apoptotic-Like Death Is Inhibited by the mazEFMediated Death Pathway Ariel Erental, Idith Sharon, Hanna Engelberg-Kulka* Department of Microbiology and Molecular Genetics, Institute for Medical Research Israel-Canada, The Hebrew University-Hadassah Medical School, Jerusalem, Israel

• Molecular Microbiology (2006) First published online 7 November 2005

• Journal compilation © 2005 Blackwell Publishing Ltd• Blackwell Science, LtdOxford, UKMMIMolecular Microbiology0950-

382X© 2005 The Authors; Journal compilation © 2005 Blackwell Publishing Ltd

• Original Article• MazG – PCD regulator in E. coliM. Gross, I. Marianovsky and G.• Glaser• MazG – a regulator of programmed cell death in Escherichia coli

Miryam Gross, Irina Marianovsky and Gad Glaser* Department of Cellular Biochemistry and Human Genetics, The Hebrew University –Hadassah Medical School, Jerusalem, Israel.

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