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14 Mycobacterial Sigma Factors and Surface Biology
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Sigma factors are interchangeable subunits of bacterial RNA polymerase that are required for promoter selectivity and transcription initiation. Sigma factors regulate processes involved in cell surface biology in many bacterial species. The characterization of stress response systems used by bacteria to respond to surface stress is extremely important because it can contribute to a better understanding of both bacterial sensing and signaling through different cell compartments, and cell envelope physiology and biogenesis. Cell integrity stress, like other forms of stress, may lead to increased expression of chaperone/heat shock genes, and the activity of these heat shock proteins (Hsps) is likely to be essential for maintaining cell envelope homeostasis. Mpt53 is a secreted DsbE-like protein containing a thioredoxin-active site. This protein was recently characterized and hypothesized to be a functional homologue of DsbA, able to catalyze the formation of disulfide bonds in unfolded secreted proteins. Even though at the current state of knowledge it is difficult to assign many of the sigma factors specific functions, one can hypothesize that some of them, such as σF, σD and σE, regulate genes responsible for the maintenance of surface homeostasis following damage that can occur during stationary phase or following exposure to surface damaging host effector molecules. Others such as σL and σM appear to be involved in the regulation of the composition of the mycobacterial surface in response to still unknown stimuli and can play an important role in modulating Mycobacterium tuberculosis interactions with the host.
Regulated intramembrane proteolysis (RIP) of RseA in E. coli. Two proteases, S1P and S2P, act sequentially in processing the transmembrane anti-sigma factor RseA. These cleavage events release the cytoplasmic domain of RseA, which is still able to bind and exert its action on σE. ClpXP, Lon and other ATP-dependent cytoplasmic proteases are involved in the final degradation of RseA and σE release.
M. tuberculosis sigma factors