Bacterial Physiology (Micr430) Lecture 15 Bacterial Physiological Adaptation (Text Chapter: 18.1; 18.5; 18.7) ote: we are switching the order of opics for Lectures 15 & 16
Jan 20, 2016
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