A small bacterial RNA regulates a putative ABC transporter.

A small noncoding bacterial ribonucleic acid of 62-64 nucleotides, RydC, was identified in the genomes of Escherichia coli, Salmonella, and Shigella. In vivo, RydC binds to the RNA-binding protein Hfq, and it is unstable when Hfq is absent. Mobility assays reveal that complex formation between RydC and Hfq is specific, with an apparent binding constant of approximately 300 nm. Sequence alignments combined with structural probing demonstrate that RydC folds as a pseudoknot. Hfq binds the loops crossing the deep and shallow grooves of the pseudoknotted RNA and reorganizes its overall conformation. An interaction with a polycistronic mRNA, yejABEF, which encodes a putative ABC transporter, was detected by affinity purification of immobilized RNA-Hfq complexes. In vivo, the yejABEF operon is expressed on minimal medium. Remarkably, its expression is reduced when RydC is absent, and the operon is degraded when RydC expression is stimulated. This observation correlates with the growth defects associated with a stimulation of its expression in vivo, generating a thermosensitive phenotype that affects growth on minimal media supplemented with glycerol, maltose, or ribose. We conclude that RydC regulates the yejABEF-encoded ABC permease at the mRNA level. This small RNA may contribute to optimal adaptation of some Enterobacteria to environmental conditions.