GROWTH OF THE STRESS-BEARING AND SHAPE-MAINTAINING MUREIN SACCULUS OFESCHERICHIA-COLI

To withstand the high intracellular pressure, the cell wall of most ba cteria is stabilized by a unique cross-linked biopolymer called murein or peptidoglycan. It is made of glycan strands [poly-(GlcNAc-MurNAc)] , which are linked by short peptides to form a co-valently closed net. Completely surrounding the cell, the murein represents a kind of bact erial exoskeleton known as the murein sacculus. Not only does the sacc ulus endow bacteria with mechanical stability, but in addition it main tains the specific shape of the cell. Enlargement and division of the murein sacculus is a prerequisite for-growth of the bacterium. Two gro ups of enzymes, hydrolases and synthases, have to cooperate to allow t he insertion of new subunits into the murein net. The action of these enzymes must be well coordinated to guarantee growth of the stress-bea ring sacculus without risking bacteriolysis. Protein-protein interacti on studies suggest that this is accomplished by the formation of ct mu ltienzyme complex, a murein-synthesizing machinery combining murein hy drolases and synthases. Enlargement of both the multilayered murein of gram-positive and the thin, single-layered murein of gram-negative ba cteria seems to follow an inside-to-outside growth strategy. New mater ial is hooked in a relaxed state underneath the stress-bearing sacculu s before it becomes inserted upon cleavage of covalent bonds in the la yer(s) under tension. A model is presented that postulates that mainte nance of bacterial shape is achieved by the enzyme complex copying the preexisting murein sacculus that plays the role of a template.