A FAMILY OF LYSOZYME-LIKE VIRULENCE FACTORS IN BACTERIAL PATHOGENS OFPLANTS AND ANIMALS

We describe a conserved family of bacterial gene products that include s the VirB1 virulence factor encoded by tumor-inducing plasmids of Agr obacterium spp., proteins involved in conjugative DNA transfer of broa d-host-range bacterial plasmids, and gene products that may be involve d in invasion by Shigella spp. and Salmonella enterica. Sequence analy sis and structural modeling show that the proteins in this group are r elated to chicken egg white lysozyme and are likely to adopt a lysozym e-like structural fold. Based on their similarity to lysozyme, we pred ict that these proteins have glycosidase activity. Iterative data base searches with three conserved sequence motifs from this protein famil y detect a more distant relationship to bacterial and bacteriophage ly tic transglycosylases, and goose egg white lysozyme. Two acidic residu es in the VirB1 protein of Agrobacterium tumefaciens form a putative c atalytic dyad. Each of these residues was changed into the correspondi ng amide by site-directed mutagenesis. Strains of A. tumefaciens that express mutated VirB1 proteins have a significantly reduced virulence. We hypothesize that many bacterial proteins involved in export of mac romolecules belong to a widespread class of hydrolases and cleave beta -1,4-glycosidic bonds as part of their function.