Lytic cycle replicative forms of bacteriophages P1 and P1 dl: Concatemer forms

Abstract

The permutedness of bacteriophage P1 chromosomes has implied that concatemers must be lytic cycle chromosomal forms (LCF). We have partially characterized LCF of P1 and a P1 dl, employing isopycnic fractionation in CsCl-ethidium bromide gradients and alkaline sucrose gradient centrifugation. The genetic identity of LCF was established by DNA filter annealings of radioactive LCF. In alkaline sucrose gradients, concatemer strands of LCF between 2 and 10 genome units long were recognized as DNA sedimenting slower than monomer supercoils and faster than single-strand monomers. Fractionation of pulse-labeled LCF showed that by 8 min after induction, most or all of the DNA synthesis proceeds by extension of concatemers. Pulse-chase experiments showed that the concatemer strands are part of persisting LCF. The P1 dl plasmid of M. E. Rae and M. Stodolsky ( Virology, 58, 32–54, 1974 ) was found to contain 133kb, as contrasted with 90 kb for the P1 plasmid. Concatemer strands were generated in thermally induced P1 dl lysogens, and had a higher average sedimentation rate than those of P1. In the P1-infected P1 dl lysogen, the sedimentation patterns of P1 and P1 dl strands were distinct. This fact indicated that recombination between P1 and P1 dl was not the major contributor to the formation of concatemer strands.