A novel acetyltransferase regulates the RNA binding capacity of the RNA chaperone Hfq in Escherichia coli
Bacterial small regulatory RNAs (sRNAs) regulate gene expression by base-pairing to their target mRNAs. In Escherichia coli and many other bacteria, this process is dependent on the RNA chaperone Hfq, which binds sRNAs and mRNAs on different faces. YhbS (renamed here as HqbA), a putative Gcn5-related N-acetyltransferase (GNAT), was identified as a novel silencer of sRNA signaling in a genomic library screen. Here, we studied how HqbA regulates sRNA signaling and determined its physiological roles in modulating Hfq activity. Using fluorescent reporter assays, we found that HqbA overproduction suppresses all tested Hfq-dependent sRNA signaling. Chromosomal HqbA suppresses the signaling of the ChiX sRNA when the C-terminus of Hfq was deleted. Direct interaction between HqbA and Hfq was demonstrated both in vivo and in vitro, and mutants that blocked interaction also interfered with HqbA suppression of Hfq. However, an acetylation-deficient HqbA mutant still disrupted sRNA signaling, suggesting that HqbA is bifunctional, with separate roles for regulating via Hfq interaction and via acetylation of undefined substrates. Gel shift assays indicated that HqbA strongly reduced the interaction between the Hfq distal face and low-affinity RNAs, but not high-affinity RNAs. Hfq-IP RNA-Seq in WT and chromosomal hqbA mutants led to the identification of two tRNA precursors, metZWV and proM, that were enriched in Hfq binding in the absence of HqbA interaction. Our results suggest that HqbA provides a level of quality control for Hfq by competing with low-affinity RNA binders. Overall design: There are 3 strains of E.coli K12 MG1655: wild type (WT), deletion of HbqA (dely) and D98AE102A mutant of HbqA (d98); two growth conditions: aerobic(ae) and anaerobic (an), and two extraction methods: total RNA or anti-Hfq precipitated, and each has two replicates. Paired-end sequencing were performed.