Distribution and composition of the lysis cassette of Lactococcus lactis phages and functional analysis of bacteriophage ul36 holin.

The bacteriophage lysis cassette, which comprises a lysin and a holin gene, was analyzed in 18 Lactococcus lactis phages. A muramidase motif was found in the lysins of c2-like phages, while an amidase motif was observed in the lysins of 936-like phages. Both amidase and muramidase types were detected among the P335 phages. The P335 lysins were separated into three groups based on amino acid sequence identity. A class I holin was recognized in 936-like and c2-like phages, whereas P335-like phages possess class II holins. The P335 holins were further divided into four groups based on sequence identity. Only the holins of 936-like phages contained putative dual-start motifs. The unusual lysis cassette of the highly virulent P335-like phage ul36 contains a unique holin (orf74B) upstream of a lysin which is present in several other P335-like phages. Using the lambdadelta Sthf system, we demonstrated that gpORF74B induces cell lysis at the same time as lambdadelta Sthf::S105, the effector of lambda lysis. Transcriptional analysis of ul36 lysis cassette showed that first transcripts are detected 35 min after infection of L. lactis cells. The lysis clock of phage ul36 appears to be controlled by the late expression of the holin and lysin genes.