SRA

In many gram-negative and some gram-positive bacteria small regulatory RNAs (sRNAs) that bind the RNA chaperone Hfq have a pivotal role in modulating virulence, stress responses, metabolism, and biofilm formation. These sRNAs recognize transcripts through base-pairing, and sRNA-mRNA annealing consequently alters the translation and/or stability of transcripts leading to changes in gene expression. We have previously found that the highly conserved 3'-to-5' exoribonuclease polynucleotide phosphorylase (PNPase) has an indispensable role in paradoxically stabilizing Hfq-bound sRNAs and promoting their function in gene regulation in Escherichia coli. Here, we report that PNPase uniquely contributes to the degradation of specific mRNA cleavage products, the majority of which bind Hfq and are derived from targets of sRNAs. Specifically, we found that these mRNA-derived fragments accumulate in the absence of PNPase or its exoribonuclease activity and interact with PNPase. Additionally, we show that mutations in hfq or in the seed pairing region of a sRNA eliminated the requirement of PNPase for sRNA stability. Altogether, our results are consistent with a model that PNPase degrades mRNA-derived fragments that could otherwise deplete cells of Hfq-binding sRNAs through pairing mediated decay. Overall design: Examination of mRNA expression in four samples of strains carrying wild type or mutant genes of rph and/or pnp. Additional examination of four samples from pnp wild type and mutant strains of RNA from Hfq co-immunoprecipitation input and output fractions. All samples were sequenced with two biological replicates.