The autolytic ('suicidase') system of Enterococcus hirae: from lysine depletion autolysis to biochemical and molecular studies of the two muramidases of Enterococcus hirae ATCC 9790.
Autolysis of Enterococcus hirae ATCC 9790 is the result of the action of endogenous enzymes that hydrolyze bonds in the protective and shape-maintaining cell wall peptidoglycan. It is thought that these potentially suicidal enzymes play a positive role(s) in wall growth and division and are expressed as autolysins when cell wall assembly and/or repair are inhibited. E. hirae possesses two potentially autolytic enzymes, both of which are muramidases. Although they hydrolyze the same bond as hen egg-white lysozyme, both are high-molecular-mass, complex enzymes. Muramidase-1 is synthesized as a zymogen, requiring protease activation. It is a glucoenzyme that is also multiply nucleotidylated with an unusual nucleotide, 5-mercaptouridine monophosphate. Muramidase-2 is almost certainly a product of a separate gene. The deduced amino acid sequence of a cloned gene for extracellular muramidase-2 showed several unusual features. It appears to be a two-, or perhaps three-domain protein with a putative glycosidase-active site near the N-terminal end and six 45-amino-acid-long repeats at the C-terminal end which are presumed to be involved with high-affinity binding to the insoluble peptidoglycan substrate. Muramidase-2 binds penicillin with low affinity. The presence of several amino acid groupings characteristic of serine-active site beta-lactam-interactive proteins is consistent with the possible presence of a penicillin-binding, third domain. Indirect evidence consistent with a role(s) for these enzymes in cell wall growth and division has been obtained. However, proof of such role(s) awaits modern genetic, molecular, and biochemical analyses.