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Expression of IroN, the salmochelin siderophore receptor, requires mRNA activation by RyhB small RNA homologues
Author(s) -
Balbontín Roberto,
Villagra Nicolás,
Pardos de la Gándara Maria,
Mora Guido,
FigueroaBossi Nara,
Bossi Lionello
Publication year - 2016
Publication title -
molecular microbiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.857
H-Index - 247
eISSN - 1365-2958
pISSN - 0950-382X
DOI - 10.1111/mmi.13307
Subject(s) - biology , ribosomal binding site , untranslated region , repressor , five prime untranslated region , translational regulation , internal ribosome entry site , messenger rna , ribosome , rna , transfer rna , translation (biology) , genetics , start codon , upstream open reading frame , gene , microbiology and biotechnology , gene expression
Summary The iroN gene of Salmonella enterica and uropathogenic Escherichia coli encodes the outer membrane receptor of Fe 3+ ‐bound salmochelin, a siderophore tailored to evade capture by the host's immune system. The iroN gene is under negative control of the Fur repressor and transcribed under iron limiting conditions. We show here that transcriptional de‐repression is not sufficient to allow iroN expression, as this also requires activation by either of two partially homologous small RNAs (sRNAs), RyhB1 and RyhB2. The two sRNAs target the same sequence segment approximately in the middle of the 94‐nucleotide 5′ untranslated region (UTR) of iroN mRNA. Several lines of evidence suggest that base pair interaction stimulates iroN mRNA translation. Activation does not result from the disruption of a secondary structure masking the ribosome binding site; rather it involves sequences at the 5′ end of iroN 5′ UTR. In vitro ‘ toeprint’ assays revealed that this upstream site binds the 30S ribosomal subunit provided that RyhB1 is paired with the mRNA. Altogether, our data suggest that RyhB1, and to lesser extent RyhB2, activate iroN mRNA translation by promoting entry of the ribosome at an upstream ‘standby’ site. These findings add yet an additional nuance to the polychromatic landscape of sRNA‐mediated regulation.