DESCRIPTION: The development of the flagellar organelle in Salmonella typhimurium is a complex process in which the assembly pathway is intimately coupled to the regulation of gene expression and levels of subunit proteins to be assembled. The transcription of the flagellar filament protein is coupled to the completion of a basal structure, the hook-basal body. We have obtained evidence of post-transcriptional control of both the filament (FliC) and hook (FIgE) subunit proteins in response to flagellar assembly. The process of extracellular assembly is mediated by secretion through a mechanism commonly used to secrete pathogenecity proteins into the cytosol of eukaryotic host cells, the type III secretion system. We have evidence that the F1gE hook component of the bacterial flagella is regulated at the post-transcriptional level in response to the assembly of the cytoplasmic component of the flagellar-specific-type III secretion apparatus, the C-rod. We also have evidence that the intracellular level of FliC filament protein is regulated by a post-transcriptional mechanism in addition to the well-characterized transcriptional control. This research proposed here will determine the mechanism (i.e., trans-acting proteins and cis-acting mRNA signals) which mediate the regulation of FlgE and F1iC levels in response to flagellar assembly. This research will also identify the cis- and trans-acting factors or signals that mediate the regulation of FlgE and FliC levels