Dissection of ΣE-dependent cell lysis in Escherichia coli: roles of RpoE regulators RseA, RseB and periplasmic folding catalyst PpiD

To understand the mechanism of Sigma(E)-dependent cell lysis, we examined the consequences of deletion derivatives of rpoE regulators rseA, rseB and rseC on Sigma(E) transcription, on levels of free versus membrane-bound Sigma(E) and on OMP-biogenesis limiting factor(s) that could impact cell lysis. RT-PCR showed that individual nonpolar delta rseA and delta rseB increased the rpoE expression to varying extents, with pronounced induction in delta rseA. Significantly the ratio of soluble (free) versus membrane-bound form of RpoE increased in delta rseA, however without increase of its total amount, unraveling furthermore complexity in RpoE regulation. Significant characteristics of cell lysis, accompanied by a severe reduction in the levels of periplasmic OMP-folding factor (PpiD), were observed in delta rseA. The cell-lysis phenotype of delta rseA was suppressed by either rseA or ppiD plasmids, but neither by rseB nor by rseC clones. However, the cell lysis of the wild-type strain was almost completely repressed not only by the rseA clone but also by the rseB clone, suggesting RseB might be limiting in vivo. Thus, increase in the ratio of free Sigma(E) in rseA mutants with a concomitant reduction in PpiD levels can account for Sigma(E)-dependent lysis in concert with a potential role of small RNAs on the lysis process.