An important role in the regulation of protease activity in living systems is played by their propeptides and endogenous protein inhibitors. Both are capable of repressing enzyme activity, ensuring it occurs only at the right time and/or in the right place. Therefore, propeptides and inhibitors may appear to perform the same function. However, many proteases have both, but the interplay between these two modes of regulation remains virtually unexplored. We investigated the role of the propeptide of Photorhabdus laumondii protealysin-like protease S (PrtS) and its emfourin-like inhibitor (ELI), photorin, in controlling intracellular enzyme activation in an Escherichia coli model. In the absence of the inhibitor, PrtS was found to be activated within cells due to autocatalytic cleavage of the propeptide, which led to the death of most bacteria. Along with this, a bacterial population emerges in which PrtS gene transcription is suppressed apparently due to the action of epigenetic mechanisms. Thus, the propeptide alone is unable to suppress aberrant intracellular activity of PrtS, whereas photorin protects cells from the toxic effect of PrtS and prevents inactivation of its gene. In summary, using PrtS as an example, we demonstrated for the first time that protealysin-like proteases (PLPs) which are ubiquitous in bacteria and are probably involved in pathogenesis, are toxic to bacteria. In this context, the function of ELIs, whose genes in bacterial genomes are co-localized with those of PLPs, is to protect bacteria from aberrant intracellular activity of PLPs.
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