The bacterium Caulobacter crescentus undergoes a simple cellular differentiation within each cell cycle. Cell division generates two distinct cell types: a motile swarmer cell and a sessile stalked cell. These two cell types differ with respect to their relative programs of gene expression and DNA replication. The generation of asymmetry upon cell division is a fundamental aspect of development in diverse organisms, including Drosophila, nematodes and fungi. In progeny stalked cells, the components of the polar flagellum are expressed under cell cycle control and assembled at the pole of the predivisonal cell that lies opposite the stalk. Two well-defined developmental checkpoints regulate flagellar morphogenesis. First, the transcription of early flagellar genes is triggered by an unknown cell cycle event that is linked to the initiation of DNA replication. Later in the cell cycle, the transcription of late flagellar genes is activated by a cell division event and the assembly of early flagellar structures. We propose that these events result in the compartmentalized transcription of both late and early flagellar genes. The overall objectives of this proposal are to define the mechanisms that couple early flagellar assembly to cell-type-specific expression of late flagellar genes. The proposed experiments, for the most part, focus on the activation of the response regulator transcription factor, FlbD, and its role in regulating temporal and spatial transcription. In addition, in order to explore cell-type specific translation, we describe experiments designed to identify the factors that regulate FlbT and the translation of flagellin genes. Finally, w e propose to identify genes outside of the flagellar regulon that are also controlled by FIbD. In order to accomplish these goals the biochemical properties of constitutive alleles of FlbD will be characterized and their effect on temporal and spatial transcription will be analyzed. In addition, the FlbD kinase will be identified. The role of the novel regulator FliX in coupling flagellum morphogenesis to gene expression will be determined by dissecting the mechanisms underlying FliX-mediated repression and activation of FlbD, and determining how the assembly of a flagellar structure regulates FbX activity. In order, to investigate the role of posttranscriptional regulation of cell-type-specific gene expression, proteins that regulate FlbT activity will be identified. Finally, DNA microarray gene expression assays will be conducted in order to identify critical cell cycle-related genes that are regulated by FIbD. The role of these genes in cell cycle progression will then be analyzed.
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