Characterization of the myo-inositol utilization island of Salmonella enterica serovar Typhimurium.

Knockout mutation of STM4432 resulted in a growth-deficient phenotype of Salmonella enterica serovar Typhimurium in the presence of myo-inositol (MI) as the sole carbon source. STM4432 is part of a 22.6-kb genomic island which spans STM4417 to STM4436 (genomic island 4417/4436) and is responsible for MI degradation. Genome comparison revealed the presence of this island in only six Salmonella strains and a high variability of the iol gene organization in gram-negative bacteria. Upon nonpolar deletion of 11 island loci, the genes involved in six enzymatic steps of the MI pathway were identified. The generation time of S. enterica serovar Typhimurium in minimal medium with MI decreases with higher concentrations of this polyol. Reverse transcriptase PCR showed five separate transcriptional units encompassing the genes iolA-iolB, iolE-iolG1, iolC1-iolC2, iolD1-iolD2-iolG2, and iolI2-iolH. Luciferase reporter assays revealed a strong induction of their promoters in the presence of MI but not glucose. The main regulator, IolR, was identified due to a reduced lag phase of a strain mutated in STM4417 (iolR). Deletion of iolR resulted in stimulation of the iol operons, indicating its negative effect on the iol genes of S. enterica serovar Typhimurium in rich medium at a transcriptional level. Bandshift assays demonstrated the binding of this putative repressor to promoter sequences of iolA, iolC1, and iolD1. Binding of IolR to its own promoter and induced iolR expression in an IolR-negative background demonstrate that its transcription is autoregulated. This is the first characterization of MI degradation in a gram-negative bacterium, revealing a complex transcriptional organization and regulation of the S. enterica serovar Typhimurium iol genes.