A Translation-Aborting Small Open Reading Frame in the Intergenic Region Promotes Translation of a Mg 2+ Transporter in Salmonella Typhimurium

Bacterial mRNAs often harbor upstream open reading frames (uORFs) in the 5' untranslated regions (UTRs). Translation of the uORF usually affects downstream gene expression at the levels of transcription and/or translation initiation. Unlike other uORFs mostly located in the 5' UTR, we discovered an 8-amino-acid ORF, designated , in the intergenic region between the virulence gene and the Mg transporter gene in the operon. Translation of promotes downstream Mg transporter expression at the level of translation by releasing the ribosome-binding sequence of the gene that is sequestered in a translation-inhibitory stem-loop structure. Interestingly, Asp2 and Glu5 codons that induce ribosome destabilization are required for -mediated translation. Moreover, the Asp and Glu codons-mediated translation is counteracted by the ribosomal subunit L31 that stabilizes ribosome. Substitution of the Asp2 and Glu5 codons in decreases MgtB Mg transporter production and thus attenuates virulence in mice, likely by limiting Mg acquisition during infection. Translation initiation regions in mRNAs that include the ribosome-binding site (RBS) and the start codon are often sequestered within a secondary structure. Therefore, to initiate protein synthesis, the mRNA secondary structure must be unfolded to allow the RBS to be accessible to the ribosome. Such unfolding can be achieved by various mechanisms that include translation of a small upstream open reading frame (uORF). In the intracellular pathogen serovar Typhimurium, translation of the Mg transporter gene is enhanced by an 8-amino-acid upstream ORF, namely, , that harbors Asp and Glu codons, which are likely to destabilize ribosome during translation. Translation of the ORF promotes the formation of a stem-loop mRNA structure sequestering anti-RBS and thus releases the RBS. Because -mediated MgtB Mg transporter production is required for virulence, this pathogen seems to control the virulence determinant production exquisitely via this uORF during infection.