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Apical growth

A new mechanism for directing bacterial cell wall growth

Growth and division of the bacterial cell wall is organized by cytoskeletal elements in the cytoplasm. Bacterial tubulin, FtsZ, forms a cytokinetic ring that directs formation of the septum that divides the cell. To become rod-shaped and grow in size, the cell has to elongate. In the most studied bacteria (e.g. Escherichia coli), this is orchestrated by bacterial actin, MreB, that forms a helical cytoskeleton and via interactions with membrane proteins organizes the machinery that inserts new peptidoglycan units into the lateral cell wall. Importantly, the phylumActinobacteria (a major phylogenetic lineage of bacteria) differs strikingly from this pattern. Most of these organisms are rod-shaped or filamentous, but do not contain or use MreB proteins. Instead, they grow apically by assembling new cell wall only at cell poles (Fig. 1). We are investigating this hitherto unclear mechanism for directing MreB-independent polarized growth in Actinobacteria.

Incorporation of new cell wall material during cell elongation

Incorporation of new cell wall material during cell elongation in Escherichia coli (left) andStreptomyces coelicolor (right). Zones of cell wall assembly are shown in blue.

The molecular basis for cell polarity and apical growth in Streptomyces

Streptomycetes form hyphae that grow by tip extension, which means that the cell wall is built at the hyphal tips (Fig. 1 and 2). The essential DivIVA protein is specifically localized to each growing tip (Fig. 2), and has a strong impact on growth and cell shape (Flärdh, 2003a, 2003b). DivIVA appears to act as a landmark protein that can establish cell wall synthesis at new sites (Hempel et al.). This suggests that DivIVA organizes the machinery for cell wall assembly at the hyphal tips in Streptomyces, thus having an analogous function to that of MreB in other bacteria. Our current work aims at clarifying the mechanisms by which DivIVA orchestrates apical growth in Streptomyces and other Actinobacteria, and how this is regulated.

Visualization of apical growth of the cell wall

Visualization of apical growth of the cell wall and localization of the cell polarity determinant DivIVA in hyphae of Streptomyces coelicolor. The sites of cell wall assembly were stained with fluorescently labelled vancomycin (red). The apical localization of DiviVA was visualized using a DivIVA-EGFP fusion (green). The image shows a spore from which two hyphae are growing out. Size bar, 6 micrometer.

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