The small RNA RssR regulates myo-inositol degradation by Salmonella enterica.

Small noncoding RNAs (sRNAs) with putative regulatory functions in gene expression have been identified in the enteropathogen Salmonella enterica serovar Typhimurium (S. Typhimurium). Two sRNAs are encoded by the genomic island GEI4417/4436 responsible for myo-inositol (MI) degradation, suggesting a role in the regulation of this metabolic pathway. We show that a lack of the sRNA STnc2160, termed RssR, results in a severe growth defect in minimal medium (MM) with MI. In contrast, the second sRNA STnc1740 was induced in the presence of glucose, and its overexpression slightly attenuated growth in the presence of MI. Constitutive expression of RssR led to an increased stability of the reiD mRNA, which encodes an activator of iol genes involved in MI utilization, via interaction with its 5'-UTR. SsrB, a response regulator contributing to the virulence properties of salmonellae, activated rssR transcription by binding the sRNA promoter. In addition, the absence of the RNA chaperone Hfq resulted in strongly decreased levels of RssR, attenuated S. Typhimurium growth with MI, and reduced expression of several iol genes required for MI degradation. Considered together, the extrinsic RssR allows fine regulation of cellular ReiD levels and thus of MI degradation by acting on the reiD mRNA stability.

The orphan regulator ReiD of Salmonella enterica is essential for myo-inositol utilization.

In Salmonella enterica serovar Typhimurium (S. Typhimurium), the genomic island GEI4417/4436 is responsible for the utilization of myo-inositol (MI) as carbon and energy source. Here, we report the characterization of a novel, island-encoded positive autoregulator termed ReiD (STM4423) that is specific to certain S. enterica strains and Escherichia coli strain ED1a able to use MI. ReiD was essential for growth with this polyol and also contributed to S. Typhimurium proliferation in swine caecum content. Providing higher copy numbers of ReiD reduced the long lag phase of 2 days during growth of S. Typhimurium in MI medium by 50%. In a heterologous host, expression of ReiD activated the transcription from the promoter of iolE/iolG, whose products catalyse the initial two steps in MI degradation. Episomal expression of iolE/iolG1 rescued the otherwise zero growth phenotype of a reiD deletion mutant in MI medium. Gel mobility shift assays with purified ReiD demonstrated directed interaction of ReiD with its own promoter and that of iolE. The repressor IolR bound the reiD promoter, implying that reiD is part of the IolR regulon. Taken together, the regulator ReiD is a trigger to accelerate the switch from more easily accessible nutrients to MI utilization by S. Typhimurium.