Summary: Monocins in were induced by UV-irradiation of liquid cultures, and defective phage particles were purified from the lysates. Electron microscopy showed flexible, non-contractile bacteriophage-tail-like particles, consisting of specific proteins of molecular mass 20-45 kDa and pl 4.6-6.7. These particles were able to lyse listerial cells. DNA sequence homologies between chromosomal DNA of monocin-producing strains and labelled phage DNAs were inferred from DNA/DNA hybridizations, suggesting that most of the prophage DNA is still present in the listerial chromosome. An endolysin gene was cloned from listerial chromosomal DNA and was identified by its expression of lytic activity against cells in a bioassay. The gene consists of 864 nt encoding a protein of 287 aa with a calculated molecular mass of 32975 Da (CPL2438). This is in good agreement with the size of a protein observed in SDS-PAGE after overexpression of the lytic protein in . The nucleotide sequence of a putative holin gene (, 291 nt) upstream of was determined after PCR-amplification of listerial DNA and it shows typical features common to the holin gene family. Expression of the encoded protein (HOL2438, 95 aa, 10.1 kDa) in was found to be lethal for the host cells. The results underline the close relationship between monocins and intact bacteriophages, indicating that monocins are incompletely assembled phage particles derived from cryptic prophages of , probably including the phage lysin.


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