Mechanism of positive regulation by DsrA and RprA small noncoding RNAs: pairing increases translation and protects rpoS mRNA from degradation.

Small noncoding RNAs (sRNAs) regulate gene expression in Escherichia coli by base pairing with mRNAs and modulating translation and mRNA stability. The sRNAs DsrA and RprA stimulate the translation of the stress response transcription factor RpoS by base pairing with the 5' untranslated region of the rpoS mRNA. In the present study, we found that the rpoS mRNA was unstable in the absence of DsrA and RprA and that expression of these sRNAs increased both the accumulation and the half-life of the rpoS mRNA. Mutations in dsrA, rprA, or rpoS that disrupt the predicted pairing sequences and reduce translation of RpoS also destabilize the rpoS mRNA. We found that the rpoS mRNA accumulates in an RNase E mutant strain in the absence of sRNA expression and, therefore, is degraded by an RNase E-mediated mechanism. DsrA expression is required, however, for maximal translation even when rpoS mRNA is abundant. This suggests that DsrA protects rpoS mRNA from degradation by RNase E and that DsrA has a further activity in stimulating RpoS protein synthesis. rpoS mRNA is subject to degradation by an additional pathway, mediated by RNase III, which, in contrast to the RNase E-mediated pathway, occurs in the presence and absence of DsrA or RprA. rpoS mRNA and RpoS protein levels are increased in an RNase III mutant strain with or without the sRNAs, suggesting that the role of RNase III in this context is to reduce the translation of RpoS even when the sRNAs are acting to stimulate translation.