A novel mechanism of RyeA/SraC induction under acid stress.

RyeA/SraC is a cis-encoded small RNA (sRNA), which act as an anti-toxin to RpoS-regulated RyeB toxin in Escherichia coli. Ectopic expression of RyeA was reported to diminish the RyeB accumulation by serving as a RNA trap. Lower abundance of RyeA in the early exponential growth phase turned out to be the outcome of its degradation by RNase BN/Z. In the current study, we show that RyeA is an acid stress inducible sRNA, and global stress responsive factor RpoS appeared to be inessential in RyeA induction. Although, ryeB-pphA dicistronic transcript at low pH condition was stimulated by ∼4-fold, however, RyeB population was found to be decreased by > 50% under the same condition by the decoy action of enhanced RyeA accumulation. Investigation of the mechanism of RyeA induceduction at low pH in the exponential phase, revealed that RNase BN/Z, which catabolizes RyeA in the exponential phase, appeared to be highly sensitive to low pH stress. Both mRNA and protein level of RNase BN transpired to be decreased to <10% of their initial population. The expression of RyeA under acid stress is regulated by a feed-forward mechanism to normalize the RyeB profusion.

Regulation of RyeA/SraC expression in Escherichia coli.

Small RNAs (sRNAs) play a central role in regulating almost all physiological processes in bacteria. Majority of those sRNAs base pair with their targets and modulate their expressions. RyeA, previously known as SraC in Escherichia coli, is transcribed from a DNA strand complementary to the one from which another stationary phase induced sRNA RyeB/SdsR is synthesised. RyeA and RyeB in the stationary phase constitute a toxin-antitoxin system where RyeA normalizes accumulation of RyeB toxin by acting as RNA sponge. Aside from that, no more information is known about the regulation of RyeA expression. In the current study, we have systematically investigated the regulation of RyeA expression in different growth phases, and identified that RyeA expression is regulated neither by stationary phase-specific σ-factor nor by RNA chaperon Hfq. A dual function ribonuclease RNase BN mitigate its expression in the exponential phase. Thus, deletion of rbn gene promoted the stability of RyeA in the exponential phase. Conversely, RyeB in the stationary phase act as RNA decoy leading to RyeA degradation, and consequently, the preclusion of RyeB in the E. coli genome elevated RyeA. These regulatory mechanisms will help identify the primary role of RyeA in E. coli.

PhoP induces RyjB expression under acid stress in Escherichia coli.

Bacterial small RNAs (sRNAs) play a pivotal role in post-transcriptional regulation of gene expression and participate in many physiological circuits. An ~80-nt-long RyjB was earlier identified as a novel sRNA, which appeared to be accumulated in all phases of growth in Escherichia coli. We have taken a comprehensive approach in the current study to understand the regulation of ryjB expression under normal and pH stress conditions. RpoS was not necessary for ryjB expression neither at normal condition nor under acid stress. Hfq also emerged to be unnecessary for RyjB accumulation. Interestingly, RyjB was detected as a novel acid stress induced sRNA. A DNA binding protein PhoP, a component of PhoP/Q regulon, was found to regulate ryjB expression at low pH, as the elimination of phoP allele in the chromosome exhibited a basal level of RyjB expression under acid stress. Ectopic expression of PhoP in ΔphoP cells restored the overabundance of RyjB in the cell. Overexpression of RyjB increased the abundance of sgcA transcripts, with which RyjB shares a 4-nt overlap. The current study increases our knowledge substantially regarding the regulation of ryjB expression in E. coli cell.