Bacterial Physiology (Micr430)

1. Note: we are switching the order of topics for Lectures 15 & 16 Bacterial Physiology (Micr430) Lecture 15 Bacterial Physiological Adaptation (Text Chapter: 18.1; 18.5; 18.7)

2. GLOBAL CONTROL NETWORK • A cell must coordinate many different regulatory circuits that control many aspects of cellular physiology in response to changes in the environment - global control • Global regulatory networks include sets of operons and regulons scattered around chromosome

3. Two-component systems • Bacteria sense and respond to changes in outside world primarily through a network of two-component signal transduction mechanisms • It consists of a sensor/kinase component (usually located on inner membrane) and a regulatory protein component (response regulators) located in the cytoplasm

4. Two-component Regulatory system Fig. 18.1

5. Two-component systems • Histidine kinases (HKs) have two domains, an input domain (N-terminal) and a transmitter domain (C-termina) • HK receives a signal at its input domain and autophosphorylates at a histidine residue in its transmitter domain • HK then transfers the phosphoryl group to an aspartate residue in the receiving domain of the partner response regulator

6. Two-component systems • Response regulators (RRs) also have two domains, a receiver domain (N-terminal) and an output domain (C-terminal) • After obtaining a phosphoryl group from HK, RR is activated and transmits the signal to its target via its output domain • Most of known phosphorylated RRs bind to DNA and stimulate or repress transcription of specific genes

7. Structures of histidine kinases Fig. 18.2

8. Structures of response regulator proteins Fig. 18.3

9. Two-component systems • The signaling pathway also includes a phosphatase that dephosphorylates the RRs, returning it to the nonstimulated state • The phosphatase may be the histidine kinase itself, the response regulator, or a separate protein • Additional proteins or enzymes may be needed for “two”-component systems that functions as carriers of phosphate – phosphotransferases • This phenomenon is phosphorelay

10. Response to Inorganic Phosphate Supply: The Pho Regulon • Regulon is a set of noncontiguous operons or genes controlled by a common regulator • Bacteria have evolved a signaling system to induce the formation of phosphate assimilation pathways when the supply of phosphate becomes limiting

11. Response to Inorganic Phosphate Supply: The Pho Regulon • Under low phosphate conditions, E. coli stimulates transcription of at least 38 genes (most of them in operons) involved in phosphate assimilation • PhoR is HK; PhoB is RR • Pho regulon is controlled by PhoR via PhoB • Phosphorylated PhoB activates transcription of genes in the Pho regulon

12. Pho signal transduction • Components involved are: • PstS, a periplasmic Pi binding protein • PstA, PstB and PstC, integral membrane proteins required for Pi uptake • PhoU • PhoR, detects Pi, either directly or indirectly • PhoB

13. Model for regulation of Pho regulon Fig. 18.11

14. Response to Osmotic Pressure and Temperature • When E. coli is growing in higher osmolarity or at high temperature, the synthesis of the bacterium’s slightly smaller porin channel, OmpC, increases relative to the larger OmpF channel • Smaller OmpC channel is advantageous to the cell when faced with higher osmolarity pressure

15. Response to Osmotic Pressure and Temperature • EnvZ is an inner membrane histidine kinase that is proposed to be an osmotic sensor • EnvZ is a transmembrane protein, with N-terminal end exposed to periplasm and C-terminal end exposed to cytoplasm • OmpR is the response regulator

16. Model for regulation of porin synthesis Fig. 18.12