Three genes coding for the endonuclease, methylase, and specificity subunits of a type I restriction-modification (R-M) system in theLactococcus lactis plasmid pIL2614 have been characterized. Plasmid location, sequence homologies, and inactivation studies indicated that this R-M system is most probably of type IC.

The lactococcal phage bIL41 belongs to the small isometric-headed phages of the 936 quasi-species and is resistant to the abortive infection determined by abiB. A 10.2-kb segment from this phage, in which late transcription is initiated, has been sequenced. Thirteen open reading frames (ORFs) organized in one transcriptional unit have been identified. The location of two of them and the structural features of the proteins they code for are evocative of terminase subunits. Five other ORFs specify proteins which are highly homologous to structural proteins from the closely related phage F4-1. By comparing the phage bIL41 sequence with partial sequences available for four related phages, we were able to deduce a chimerical phage map covering the middle-and a large part of the late-expressed regions. Phages from this quasi-species differ by the insertion or deletion of either 1 to about 400 bp in noncoding regions or an entire ORF. Transcription was initiated 9 min after infection at a promoter with a ؊10 but no ؊35 consensus sequence. Synthesis of a phage activator protein was needed for initiation of transcription. A large 16-kb transcript covering all of the late-expressed region of the genome was synthesized. This transcript gave rise to smaller units. One of these units most probably resulted from a RNase E processing.Lactococci are widely used for the manufacture of dairy products. During the fermentation processes, they are challenged with high concentrations of a variety of phages. These highly selective environmental conditions are most probably responsible for the selection of strains with diverse phage defense mechanisms, including adsorption interference, restriction and modification, and abortive infection (38). Lactococci represent, therefore, a good model for studying bacteria or phage coevolution and identifying phage defense mechanisms. A variety of phage abortive infection mechanisms have been described in Lactococcus lactis (2,24,25,30,32,40,54). All of them are active against phages of one of the prevailing groups in the dairy environment, designated species 936 (42). A model has been proposed for two of these mechanisms, designated AbiB (24) and AbiD1 (2). AbiB was shown to prevent phage growth by promoting a dramatic degradation of phage transcripts 10 min after infection (57). It was proposed that phage infection either induces the synthesis or stimulates the activity of an RNase in AbiB cells (57). AbiD1 was proposed to prevent phage growth by decreasing the amount of an essential phage product encoded by middle-expressed genes (9). However, the molecular basis of phage-Abi interactions has not yet been elucidated, not only because of their complexity but also because of the scarcity of data available on lactococcal phages of the 936 species.The lactococcal 936 phage species will be designated as a quasi-species in this paper, as suggested by Casjens et al. (15) for the lambdoid phages. Isometric-headed phages from the 936 quasi-species have a double-stranded DNA genome of about 30 kb and c...