Post on 13-Dec-2015
Chapter 8Lecture Outline
Transcription, Translation, and Bioinformatics
Transcription, translation
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 2
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 3
8.5Secretion: Protein Traffic Control
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 4
Protein Traffic Many bacterial proteins reside
in cytoplasm Others targeted to other sites
Plasma membrane Periplasm Gram outer membrane Secreted outside of bacterium
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 5
Protein Secretion
Movement of proteins out of cytoplasm To the periplasm
Usually secreted unfolded Require a second set of proteins for correct folding
(chaperones) To the outer membrane Across both membranes to the surrounding
environment Special export systems are required Move hydrophilic proteins through hydrophobic
membrane barriers
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 6
Protein Export to the Cell Membrane Proteins for cell membrane (e.g.cytochromes) are
tagged with very hydrophobic N-terminal signal sequences
Contain hydrophobic transmembrane spanning regions Signal recognition particle (SRP) binds to nascent N-
terminus and halt further translation Nascent protein with SRP and ribosome is delivered to
the membrane bound protein FtsY Two outcomes
Co-translational insertion into cell membrane Complete synthesis and then delivery to SecYEG translocon for
insertion
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SRP and Cotranslational Transport
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Protein SecretionAnimation: Protein Export
Click box to launch animation
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Examples are superoxide dismutase, maltose binding protein
Sec-dependent general pathway Protein completely translated in
the cytoplasm Protein captured unfolded by
SecB and delivered to SecA and the membrane-bound SecYEG
Protein is pushed by SecA through SecYEG in multiple steps
LepB cleaves signal sequence Periplasmic chaperones fold the
protein
Protein Export to Periplasm
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Examples are toxins Type I-VI secretion systems Type I are ABC (ATP Binding
cassette) transporters All have three components
ABC protein at the inner membrane
Periplasmic protein lashed to the inner membrane
Outer membrane channel Proteins secreted through Type I
never contact the periplasm Example: Hly (E. coli hemolysin)
Protein Secretion to the Outer Surroundings
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 11
Bioinformatics: Genome Analysis Genome provides raw DNA sequence
What sequences encode proteins? Open reading frames (ORFs)
What sequences control expression? Promoters
What proteins perform specific functions?
Biochemical analysis of protein function
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Evolutionary Relationships Sequence reflects ancestry Homologs
Orthologs Genes duplicated via appearance of new
species Identical function in different organisms
Paralogs Genes duplicated within a species
Perform slightly different tasks in cell Can develop new capabilities