Bacterial Physiology (Micr430)

Lecture 15Bacterial Physiological

Adaptation

(Text Chapter: 18.1; 18.5; 18.7)

Note: we are switching the order of topics for Lectures 15 & 16

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

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

Two-componentRegulatory system

Fig. 18.1

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

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

Structures of histidine kinases Fig. 18.2

Structures of response regulator proteins

Fig. 18.3

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

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

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

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

Model for regulation of Pho regulon

Fig. 18.11

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

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

Model for regulation of porin synthesis

Fig. 18.12