The flagellar hook protein, FlgE, of Salmonella enterica serovar typhimurium is posttranscriptionally regulated in response to the stage of flagellar assembly.

We investigated the posttranscriptional regulation of flgE, a class 2 gene that encodes the hook subunit protein of the flagella. RNase protection assays demonstrated that the flgE gene was transcribed at comparable levels in numerous strains defective in known steps of flagellar assembly. However, Western analyses of these strains demonstrated substantial differences in FlgE protein levels. Although wild-type FlgE levels were observed in strains with deletions of genes encoding components of the switch complex and the flagellum-specific secretion apparatus, no protein was detected in a strain with deletions of the rod, ring, and hook-associated proteins. To determine whether FlgE levels were affected by the stage of hook-basal-body assembly, Western analysis was performed on strains with mutations at individual loci encompassed by the deletion. FlgE protein was undetectable in rod mutants, intermediate in ring mutants, and wild type in hook-associated protein mutants. The lack of negative regulation in switch complex and flagellum-specific secretion apparatus deletion mutants blocked for flagellar construction prior to rod assembly suggests that these structures play a role in the negative regulation of FlgE. Quantitative Western analyses of numerous flagellar mutants indicate that FlgE levels reflect the stage at which flagellar assembly is blocked. These data provide evidence for negative posttranscriptional regulation of FlgE in response to the stage of flagellar assembly.

A HilA-independent pathway to Salmonella typhimurium invasion gene transcription.

Salmonella typhimurium invasion of nonphagocytic cells requires the expression of a type III secretion system (TTSS) encoded within Salmonella pathogenicity island 1 (SPI1). TTSS gene transcription is activated in response to environmental signals and requires transcriptional regulators encoded within (HilA) and outside (SirA) SPI1. Two unique loci, sirB and sirC, which contribute to SPI1 gene transcription were defined. sirC is an SPI1-encoded transcription factor of the AraC family that contributes to the invasive phenotype. sirB is required for maximal expression of sirC and consists of two open reading frames located near kdsA, a gene involved in lipopolysaccharide biosynthesis. sirC expression, unlike expression of other SPI1 genes, does not require HilA. Overexpression of sirC or sirA restores expression of a subset of SPI1 genes, including invF and sspC, in the absence of HilA. These data define roles for SirC and SirA as part of a HilA-independent pathway to SPI1 gene expression. We postulate that HilA-independent activation of inv expression is important for efficient assembly and function of the SPI1 TTSS.