The bacteriophage kh receptor of Lactococcus lactis subsp. cremoris KH is the rhamnose of the extracellular wall polysaccharide

Cloning of a chromosomal gene required for phage infection of Lactococcus lactis subsp. lactis C2

A phage-resistant mutant with a defect in a membrane component required for phage infections in Lactococcus lactis subsp. lactis C2 was transformed with a chromosomal library of the wild-type, phage-sensitive strain. Of the 4,200 transformants screened for phage sensitivity, three were positively identified as phage sensitive. A cause-and-effect relationship between the cloned chromosomal fragments and the phage-sensitive phenotype was established on the basis of the following two criteria: (i) the frequency of loss of the cloned fragments in the absence of antibiotic selection pressure correlated with the frequency of loss of phage sensitivity; and (ii) phage sensitivity was transferred to 100% of recipient, phage-resistant cells transformed with the cloned fragment. The cloned chromosomal DNA from the three independent isolates was physically mapped with restriction endonucleases. The sizes of the cloned fragments were 9.6, 11.8, and 9.5 kb. Each fragment contained an identical stretch of DNA common to all three, which was 9.4 kb. The gene that conferred phage sensitivity was localized by subcloning to a 4.5-kb region. Further subcloning indicated that a single EcoRI site within the 4.5-kb region must lie within the gene or its promoter. The required 4.5-kb region was sequenced and found to code for one partial and two complete open reading frames. The gene required for complementation was functionally mapped by Tn5 mutagenesis and localized to one of the two complete open reading frames, which was designated pip (an acronym for phage infection protein). pip is 2,703 bases in length. Potential promoters start 206 and 212 bases upstream of the open reading frame. A ribosome binding site and a seven-base spacer precede the GTG (Val) translation initiation codon. The amino acid sequence deduced from the gene has 901 residues and an M(r) of 99,426. Hydropathy analysis revealed four to six potential membrane-spanning regions, one near the amino terminus and the others at the extreme carboxyl terminus. The amino terminus has characteristics of a signal sequence. The putative protein would have a 650-residue, central polar domain.

A membrane protein is required for bacteriophage c2 infection of Lactococcus lactis subsp. lactis C2

Phage-resistant mutants, isolated from cultures of Lactococcus lactis subsp. lactis C2 infected with phage c2, did not form plaques but bound phage normally. The mutants were sensitive to another phage, sk1, although the number of plaques was reduced approximately 56% and the plaques were four times smaller. Binding to phage sk1 was reduced about 10%. Another group of phage-resistant mutants, isolated from cultures infected with phage sk1, bound normally to both phages c2 and sk1 but did not form plaques with either phage. Carbohydrate analyses by gas chromatography of the cell walls showed no significant differences in saccharide compositions between the wild-type and phage-resistant cells. However, a difference was observed in the interactions of the phage with the cytoplasmic membranes. Membranes from the wild-type cells, but not mutant cells, inactivated phage c2. Phage sk1 was not inactivated by membrane from either strain. Treatment of wild-type membranes with proteinase K eliminated the ability of the membrane to inactivate the phage, whereas treatment with mutanolysin had no effect. On the basis of this ability to inactivate the phage, a membrane protein was partially purified by gel filtration and ion-exchange chromatography. Under nondenaturing conditions, the phage-inactivating protein has an apparent Mr of approximately 350,000. The protein has an apparent subunit size of 32 kDa, which suggests that it normally exists as a multimer with 10 to 12 subunits or in association with other membrane components. It is proposed that this protein is required for phage c2 infection.