Signal Transduction and 'Two Component Regulatory Systems'
In prokaryotes, a wide variety of stimuli are sensed and transduced leading to altered gene expression. Important examples in E. coli, include chemotaxis, nitrogen assimilation and outer membrane protein expression. Two sets of proteins, bearing sequence similarities, are known to be involved in these regulatory systems, which are, therefore, described as 'two component regulatory systems'. The first component is a protein kinase, described as 'kinase', that autophosphorylates (using ATP) a histidine residue, and transfers its phosphoryl group to an aspartate residue of the second component described as 'regulator'. Finally, a phosphatase activity may reset the system by restoring the regulator to its unphosphorylated state. Regulator phosphate levels are controlled in response to environmental condition.
The above signal transduction, using 'two component systems' can be illustrated using the following three example.
In homogenous environment, E. coli swims smoothly, but it moves towards or away from wide variety of compounds classified as 'attractants' or 'repellents' respectively. This mechanism involves a post-translational regulation of gene expression, involving reversible phosphorylation of two proteins, CheA (kinase) and CheY (regulator).
Many species of bacteria regulate both the amount and the activity of 'glutamine synthetase' enzyme in response to nitrogen availability. The gene for this enzyme is glnA and its major promoter is glnAp2. Transcription of the gene is regulated by a two component system (NtrB = kinase, NtrC = regulator). NtrB autophosphorylates and transfers its phosphate group to NtrC, which in its phosphorylated form activates glnAp2.The phosphorylation of NtrC is regulated by nitrogen availability through another protein called 'PII'.
Outer membrane protein expression
Earlier in this section, we discussed regulation of expression of genes ompC and ompF (meant for outer membrane proteins) at the translational level using antisense RNA. Another mechanism at the transcriptional level involves a two component system (EnvZ = kinase; OmpR = regulator). EnvZ autophosphorylates and transfers its phosphate group to OmpR, which in its phosphorylated form, stimulates expression of ompF under 'low osmolarity' conditions and that of ompC under 'high osmolarity' condition. Phosphorylation of OmpR is also regulated due to hydrolysis in the joint presence of EnvZ and ATP.
For details of the above regulatory mechariisms and signal transduction, the readers are advised to consult Ann. Rev. Biochem. 60 : 401-441 (1991).