Cloning, molecular characterization, and expression of the genes encoding the lytic functions of lactococcal bacteriophage phi LC3: a dual lysis system of modular design.

The genes encoding the lysis proteins of Lactococcus lactis bacteriophage phi LC3 were cloned, sequenced, and expressed in Escherichia coli. The phi LC3 lysis genes, lysA and lysB, encode a membrane-disrupting protein (LysA) of 88 amino acids, and a cell wall degrading protein (LysB) of 429 amino acids, which shares significant sequence similarity with lysins from the Streptococcus pneumoniae phages Cp-1, Cp-7, and Cp-9, and Lactobacillus delbrueckii phage mv1. Both LysA and LysB function in E. coli, as judged by lysis of the E. coli host cells and by lytic activity against lactococcal cells when the cloned lysA and lysB genes are expressed. The LysA protein possesses two putative transmembrane helices and highly charged N- and C-termini, and is structurally similar to phage holins that are known to induce lesions in the inner membrane through which phage endolysin can be released to its cell wall substrate. The C-terminal end of LysB contains two highly homologous sequence repeats of 43 amino acids. The LysB repeats show strong sequence similarity to repeats found in lytic enzymes from other Gram-positive bacteria and from Bacillus subtilis phage phi 29 and PZA, as well as in some functionally unrelated proteins, and they are possibly involved in binding of the enzyme to the cell wall substrate. The organization of the dual phi LC3 lysis system supports earlier suggestions that exchange of modular units is an important principle in protein evolution.