Genetic and physical characterization of P1 dlw prophage and its derivatives

Abstract

P1 dlw, a defective derivative of coliphage P1, carries the chromosomal lac region. Marker rescue for 10 different P1 alleles distributed along the known P1 genetic map indicates that the P1 dlw prophage probably carries the entire P1 genome in addition to the chromosomal lac region. Results of genetic crosses are compatible with insertion of the lac region between the 36 and vir loci, suggesting 34–36-lac-35 linkages. The molecular weight of the P1 dluw prophage, as determined by electron microscopical analysis, is 103.2 ± 5.1 × 10 6. The excision of lac from P1 dlw produces two P1-immune lac − segregant types. Type I carries all 10 markers tested with a distinct defect between genes 1 and 34. The molecular weight of this defective prophage is 63.7 ± 2.4 × 10 6, larger by 2.8 × 10 6 than P1 prophage. The second lac − segregant type (Type II) is deleted for genes 34, 36, and 35. The molecular weight of the defective Type II prophage is 51.1 ± 1.5 × 10 6. The two segregation patterns as well as their frequency of occurrence are independent of the RecA pathway. Marker rescue for P1 genes in the vicinity of inserted bacterial DNA appears to depend on the bacterial RecA pathway. pIH1972, a lac + plasmid derived from P1 dlw, has a molecular weight of 23.6 ± 1.0 × 10 6 and no rescuable PI markers but is incompatible with P1 prophage. It is assumed that the 48.4 × 10 6 chromosomal IS3-bounded lac segment or a major part of this segment is integrated in P1 to produce P1 dlw, which is 42.3 × 10 6 larger than the P1 prophage. The frequency and the pattern of the recA-independent segregation of lac suggest that the chromosomal insertion may carry IS-like elements.