Eine Frage der Form: Mechanismen morphologischer Differenzierung in Bakterien

Prokaryotes have evolved a diversity of cell shapes and lifestyles, but the mechanisms that underlie this variety are largely unknown. There are two major determinants controlling the morphology of bacteria. The cytoskeleton plays a key role in the temporal and spatial organization of the prokaryotic cell and thus defines the appearance of the organism, wheras the cell envelope, composed of peptidoglycan, protects the cell from lysis due to pressure differences and helps to maintain its shape. In this study a new class of polymer-forming proteins, termed bactofilins, was identified. Proteins belonging to this group are widely conserved among bacteria. In the dimorphic bacterium Caulobacter crescentus, the two bactofilin paralogues BacA and BacB cooperate to form a sheet-like structure lining the cytoplasmic membrane in proximity of the stalked cell pole. These assemblies mediate polar localization of a peptidoglycan synthase involved in stalk morphogenesis, thus complementing the function of the actin-like cytoskeleton and the cell division machinery in the regulation of the cell wall biogenesis. In other bacteria, bactofilins can establish rod-shaped filaments or associate with the cell division apparatus indicating considerable structural and functional flexibility. All bactofilins investigated so far polymerize spontaneously in the absence of additional cofactors in vitro, forming stable filament bundles. These results suggest that the observed structures have evolved as an alternative to intermediate filaments, serving as versatile molecular scaffolds in a variety of cellular pathways. During phosphate starvation C. crescentus cells transiently change their shape. The most prominent feature of this differentiation process is a dramatic elongation of the stalk to produce appendages as much as 30 times longer than those of cells growing in phosphate-rich medium. Using microarray analysis, the response of cells to a limitation of phosphate was examined in more detail. This study revealed many similarities to the well-characterized reaction of E. coli and identified putative members of the Pho-regulon in C. crescentus, including genes that encode enzymes such as phosphatases and endonucleases. In addition, the so far poorly investigated type II secretion system of this organism was induced under phosphate starvation. This observation is consistent with the increased expression of genes involved in the release of phosphate from organic compounds that cannot be imported into the cell due to size limitations. The transport of the respective enzymes to the cell surface may be a possibility to capture phosphate from sources that are otherwise not accessible. The putative lipoprotein CC0170 is associated with the type II secretion systems and may act as a chaperon for parts of the secretion machinery or for some transported compounds. The transcriptional profiling revealed that bacA is up-regulated during phosphate limitation. Deletion of this gene leads to short stalks in phosphate-low media. Thus, elongation of the stalk under these conditions seems to be particularly dependent on bactofilins, suggesting an key role of these proteins in cellular differentiation.