Molecular characterization of an autolytic amidase of Listeria monocytogenes EGD

The gene encoding a 102 kDa autolysin has been cloned from an expression library of Listeria monocytogenes EGD genomic DNA, using a direct screening protocol. The encoded protein has two domains, an N-terminal enzymic domain showing a high level of homology to the amidase domain of the major autolysin (atl) of Staphylococcus aureus, and a C-terminal, putative cell-wall-binding domain containing four imperfect direct repeats. In order to examine the role of the enzyme, the autolysin-encoding gene was insertionally inactivated by site-specific integration of a temperature sensitive plasmid. The enzyme accounts for 66% of the total lytic enzyme activity when L. monocytogenes walls are used as substrate and several of the major autolytic bands are missing on renaturing gels when compared to the wild-type. The enzyme does not appear to be directly involved in cell separation but has a role in motility. Characterization of the recombinant enzyme expressed in Escherichia coli has revealed it to be an amidase and to be able to hydrolyse a range of peptidoglycan substrates.