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Bacterial Physiology (Micr430)

Bacterial Physiology (Micr430). Lecture 17 Bacterial Responses to Environmental Stress (Text Chapter: 19). Environmental Stress and Response.

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Bacterial Physiology (Micr430)

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  1. Bacterial Physiology (Micr430) Lecture 17 Bacterial Responses to Environmental Stress (Text Chapter: 19)

  2. Environmental Stress and Response • Bacteria respond in a variety of physiological ways to environmental stresses such as changes in pH, temperature, food source, radiation damages and oxidative toxicity. • We will describe: • Heat-shock response • DNA damage response • SOS response

  3. Thermal Stress and Heat Shock Response • Upon a shift from 30º to 42º C, E. coli and other bacteria transiently increase the rate of synthesis of a set of proteins called heat shock proteins (HSPs). • Many of these HSPs are required for cell growth or survival at more elevated temperatures (thermal-tolerance). • Heat shock proteins are classified based on molecular weights • Hsp70 family (DnaK) • Hsp60 family (GroEL) • Hsp40 family (DnaJ)

  4. Heat Shock Response • In E. coli, after a temperature upshift (32 ºC to 42 ºC), there is a transient increase in the amount of sigma factor σ32, also called σH or RpoH. • σ32 recognizes promoters of genes in a major heat shock regulon – the σ32 regulon. • During growth at 30 ºC, σ32 can be degraded by several proteases. However, if σ32 is bound to RNAP, it is protected from degradation.

  5. Heat Shock Response • Cis-acting mRNA sites within the 5’ region of rpoH (σ32 or σH ) message form temp sensitive secondary structures that sequester the ribosomal binding site. • At higher temperature, these secondary structures melt, thereby enabling more efficient translation of the rpoH message.

  6. Heat Shock Response • The cell uses the DnaK-DnaJ-GrpE chaperon team to interact with σ32 at low temperature, sequestering σ32 from RNAP. • Failure to bind RNAP facilitates degradation of the σ32 factor. • Upon heat shock, there is an increase in the number of other unfolded or denatured proteins that can bind to DnaK or DnaJ. This reduces the level of free DnaK/DnaJ molecules available to bind σ32, allowing σ32 to bind RNAP, which protects it from degradation.

  7. (From Moat book)

  8. Functions of E. coli Hsps • Fold newly synthesized proteins at all temperatures • Export proteins at all temperatures • Refold misfolded polypeptides • Perform proteolysis of improperly folded or otherwise abnormal proteins

  9. DNA Damage Repair • UV-damaged DNA: by photoreactivation • Damaged DNA: by nucleotide excision repair • Damaged DNA: by base excision repair

  10. Repairing UV-damaged DNA • Exposure to UV can cause formation of thymine dimers • During photoreactivation, a special enzyme called DNA photolyase reverses the dimerization reaction upon absorbing blue light.

  11. Nucleotide Excision Repair • In E. coli, the products of three genes, uvrA, uvrB and uvrC, encode UvrABC endonuclease • It recognizes distortion in DNA region caused by UV-induced thymine dimers and cuts both sides of the dimer • Gap is filled in by DNA pol I and sealed by DNA ligase.

  12. Repair of Deaminated Bases • A common type of DNA damage is the deamination of bases (amino group is replaced by keto group) • Deaminated bases pair with wrong bases during replication, creating mutations • To repair, deaminated bases are removed by DNA glycosylases – catalyzing breakage of the N-glycosyl bond between the base and the sugar, leaving an apyrimidinic or apurinic site. These sites are called AP sites

  13. Repair of Deaminated Bases • After the base is removed, AP endonucleases cleave the phosphodiester bond next to the AP site, usually on the 5’ side • This generates 3’-hydroxyl end, which is extended by DNA pol I as the 5’-exonuclease activity of this enzyme removes a portion of damaged DNA ahead. • DNA ligase seals the nick.

  14. SOS response • SOS is response to distress signal generated by DNA damage • This results in the induction of over 20 unlinked genes whose products are involved in repairing DNA damages, allowing DNA replication to proceed past the damaged site and stalling the cell division to give the cell time to repair damaged DNA.

  15. SOS response • SOS signal is generated when RecA protein binds to ssDNA (produced during DNA damage) • When the signal is received, the SOS regulon is induced by inactivation of a repressor LexA.

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