Enzymology of elongation and constriction of themurein sacculus of Escherichia coli
Introduction
Although final proof has never been presented up to now murein hydrolases are still widely believed to be essential for growth and division of the bacterial cell wall [1], [2], [3]. Indeed it seems quite apparent that meshes of the bag-shaped murein net that completely encloses the cell have to be cleaved to allow for the insertion of new material during growth. It is even more obvious that during cell division the murein septum has to be split by specific enzymes to promote separation of the daughter cells. This raises the question of how these potentially autolytic enzymes are controlled by the cell to avoid spontaneous bacteriolysis. Growth models have been presented that try to explain by what means the interaction of murein synthases and hydrolases may be organized to guarantee that the murein hydrolases do not turn into perilous autolysins [4], [5], [6], [7]. According to these models the murein hydrolases exclusively act as vital space- or pacemaker enzymes during cell enlargement and as splitting enzymes during cell division [8], [9].
Section snippets
Three-for-one growth mechanism
Studies of the changes in the structure of the murein that take place during growth have been performed with batch cultures as well as with synchronously growing and dividing cells [10], [11], [12]. Both pulse and pulse-chase experiments were done. The three major results of these investigations that must be explained by any growth model are as follows: first, murein is turned over with a rate of about 40–50% per generation [13], [14], [15]. Second, during cell elongation new material is
A multienzyme complex combining murein synthases and hydrolases
The three-for-one mechanism calls for the coordination of the activities of quite a number of different enzyme specifications [7]. Importantly they have to be coordinated both in time and space. Therefore we tried to obtain evidence for the existence of a multi-enzyme complex that would combine these activities in a macromolecular murein synthesizing machinery. Such a complex would consist of two opposing enzyme classes, synthases and hydrolases, and may thus most appropriately be called a
Phenotypes of mutants lacking murein hydrolases
Enzymes that cleave specific bonds in the murein are present in E. coli in great number and specificity [24]. For each type of covalent bond at least one but in most cases even several specific enzymes exist. Up to now six different lytic transglycosylases, four different amidases and three different D,D-endopeptidases have been described. The specific function of these enzymes with respect to the different processes of the cell cycle is not known. Therefore we started to construct a series of
Mechanism of enlargement and division of the murein sacculus
The structural design of murein as a net of glycan strands that are cross-linked by peptides requires the opening of meshes if the surface of the net is to be enlarged. If murein hydrolases are not involved other enzymes must fulfill this essential function. Interestingly, several of the enzymes that participate in murein metabolism catalyse transferase reactions [25], [26]. This includes the transpeptidases, the synthetic transglycosylases and also the lytic transglycosylases, although these
Growth of the murein in Gram-positive bacteria
The conception of transferases as the principle that allows enlargement of the murein net by combining the two essential steps breaking of old and making of new bonds opens up a different view of the mechanism of growth of the thick, multilayered murein of Gram-positive bacteria. It is generally believed that the multilayered peptidoglycan wall grows by the addition of new layers of murein underneath the existing ones. Removal of the outermost layers allows then that the newly added ones become
Acknowledgements
We gratefully acknowledge the help of Heinz Schwarz and Jürgen Berger in preparing the electron micrographs and Uli Schwarz for stimulating discussions and general support.
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