Genetic and phenotypic characterization of the RyhB regulon in Salmonella Typhimurium

Salmonella encodes two homologs of RyhB, a small RNA (sRNA) involved in iron homeostasis. In Salmonella Typhimurium, the expression of both RyhB-1 and RyhB-2 is negatively regulated by the Fur repressor, while stationary phase is the primary signal inducing RyhB-2 expression. To identify the target mRNAs of RyhB-1 and RyhB-2, 9 predicted target genes were analyzed by quantitative RT-PCR to monitor differential transcript levels between wild type and each of three mutants (ΔryhB-1, ΔryhB-2 and ΔryhB-1ΔryhB-2) under conditions that maximize the expression of both sRNAs. Our results, along with bioinformatic predictions, suggest that the genes acnA, sodB, ftn, STM1273.1n, and acnB are the primary targets of at least one of these sRNAs. To understand the biological roles of the RyhB regulon, the aforementioned deletions were created in either wild type or Δfur backgrounds and were subjected to various phenotypic assays. The results showed that these sRNAs are singularly or additively involved in the expression of multiple phenotypes, including acid resistance, resistance to hydrogen peroxide, and sensitivity to bactericidal antibiotics. The results support a model whereby RyhB-1 and RyhB-2 have a global regulatory effect on diverse cellular pathways in response to multiple environmental cues via post-transcriptional regulation of distinct sets of overlapping targets.