Talmudic Question #30

 

Many bacteria, for example, E. coli and B. subtilis, re­gulate their gene expression via a large number of distinct devices that operate on almost all conceivable biochemical levels. Why so many?

 
 

 
 

Comments

[Celia M. Ebrahimi]

Bacteria possess a multitude of regulatory proteins (ex. kinase, phosphatases just to name a couple) that are responsible for ensuring their ability to control their structure, function, differentiation and adaptation to environmental conditions. In addition many of these regulatory proteins have overlapping functions with other regulatory proteins. This leads one to ask the obvious question – why would a bacterium want so many regulatory proteins and why have regulatory proteins with overlapping functions?
The second half of this question is easy to answer. Having two regulatory proteins with overlapping functions ensures that the proper regulation continues to occur even in the absence of one of the regulatory proteins. For example the synthesis of (p)ppGpp is regulated by two proteins: RelA and SpoT. In the absence of RelA the synthesis of (p)ppGpp still occurs and is still detected in the cell thanks to SpoT. The overlapping function of SpoT ensures that (p)ppGpp expression remains “normal” under nutrient rich and nutrient poor conditions, thus allowing the cell to respond accordingly. Therefore, overlapping regulatory proteins function to ensure the proper regulation of numerous cellular processes essential for bacterial survival.
The first half of the question asks, why would a bacterium want so many proteins? Numerous bacterial proteins have been implicated to perform essential functions in fairly simple tasks such as cell growth and division. One possibility for the abundance of so many bacterial proteins is to ensure that the executed task is being performed correctly. For example, each protein in the RNA transcription complex performs functions in ensuring proper gene expression (ex. alternative sigma factors recognizing elements in the promoters of stress response genes). The loss of one of these factors could have a significant negative impact on cell survival. Another reason why there are so many bacterial proteins is to ensure that bacterial tasks are executed only when required. For example, induction of the stress response pathways occurs via a signaling cascade event involving numerous proteins and resulting in a change in gene expression. Each of these proteins in the signaling cascade event could function as a checkpoint to ensure that unnecessary changes in gene expression do not occur. A final reason why bacteria have so many proteins may reflect the ability of the bacterium to “fine tune” cellular processes to allow for optimal survival. Having the ability to change a single protein in a protein complex may optimize a bacterial metabolic process thereby enhancing bacterial fitness.
Therefore, the abundance of proteins in bacteria allow the bacteria to control when and how cellular tasks are executed and to ensure that each task is performed optimally.

Celia M. Ebrahimi (Student, Integrative Microbiology course)