The lysin gene (lysA2) of the Lactobacillus casei bacteriophage A2 was cloned and expressed in Escherichia coli. LysA2 is an endopeptidase that hydrolyzes the bond between the terminal d-alanine of the peptidoglycan tetrapeptide and the aspartic acid residue that forms the bridge with the l-lysine of a neighboring peptidoglycan chain, characteristic of Gram-positive bacteria included into the A4 peptidoglycan subgroup. This includes most lactobacilli, Lactococcus lactis, Pediococcus acidilactici, and Pediococcus pentosaceus, the walls of all of which were substrates for the enzyme. Specific binding of LysA2 to the wall of these bacteria is mediated by its C-terminal moiety, does not need the N-terminal catalytic domain for recognition, and is stable: at least 88% of the molecules were still bound to L. casei after 3 days in phosphate buffer at 4A degrees C. The enzyme acts as a monomer, is active at pH values between 4 and 6, and at temperatures ranging between 18A degrees C and 50A degrees C while being independent of divalent cation addition. The enzyme showed strong resistance to incubation at high and low pH values but became progressively inactivated at 50A degrees C and above. LysA2 is bactericidal, the viability of L. casei cultures dropping to 1% in 10 min, under the standard conditions used for the enzymatic assay.