GLOBAL CONTROL OF DIFFERENTIATION IN CAULOBACTER

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How one cell differentiates to give rise to different cell types is one of the fundamental problems in biology. One way by which cell can differentiate is by cell division of an already asymmetric mother cell. These asymmetric cell divisions are common to organisms of all kingdoms, including humans, the fruit fly Drosophila, the nematode Caenorhabditis elegans, yeast, and bacteria such as Bacillus subtilis and Caulobacter crescentus. We want to understand the global controls responsible for the generation of asymmetry during cellular differentiation in Caulobacter crescentus. In this simple organism, cell division gives rise to two morphologically and functionally different progeny cells: a motile swarmer cell and a sessile stalked cell. This differentiation is due to the expression of asymmetry in a predivisional cell that has two different polar domains. The pole that will give rise to the swarmer cell has a single flagellum and the other one has a stalk. This asymmetry reflects numerous internal processes, such as the localization of proteins, cell type specific DNA replication, establishment of chromosome structure, temporally controlled DNA methylation, and localized transcription. Thus, the study of pole biosynthesis provides a convenient assay of these mechanisms. A major advantage of Caulobacter as an experimental system is the ease with which cell populations can be synchronized without perturbing the normal physiology of the cell. We will study the role of three genes involved in the global control of Caulobacter differentiation and cell division to determine how temporal and positional information are integrated in order to obtain proper differentiation. rpoN, the gene encoding the sigma-54 transcription factor, is a global regulator of pole biosynthesis and cell division, gdnA controls the positioning of polar structures and is involved in maintaining the normal axis of symmetry during cell division, and FtsZ is a GTP-binding protein involved in the initiation of cell division and of stalk biosynthesis. These genes control the expression and localization of a multitude of genes and gene products required for cell differentiation. By determining how the abundance and the localization of these global regulators is controlled during the cell cycle and how they interact, we will address the basic mechanisms of cell differentiation. This will further our knowledge of how asymmetry is generated, a question central to our understanding of the development of both prokaryotic and eukaryotis biological systems. Indeed, a number of diseases, like cancer, result from a loss of the proper control of cellular differentiation.