The structure of replicating bacteriophage T1 DNA: comparison between wild type and DNA-arrest mutant infections.

Analysis of the structure of replicating phage T1 DNA has identified three major forms of T1+ intracellular DNA; (i) concatemeric molecules with single-stranded interruptions, (ii) monomer-length DNA with single-strand interruptions, and (iii) monomer-length molecules with completely intact single strands. The interruptions in concatemeric DNA are spaced at intervals, with a mean distance equivalent to one monomer length but with a broad distribution and many are in the form of gaps. Type (iii) molecules are probably derived from mature progeny phage particles disrupted during cell lysis. Under nonpermissive conditions, cells infected with amber mutants in T1 genes 3.5 and 4 show the premature arrest of phage DNA synthesis, failure to make concatemeric DNA, and reduced genetic recombination. Intracellular DNA from gene 3.5- and gene 4- infections consist of a uniform population of molecules approximately 10-12% shorter than mature monomers. They are stable throughout infection, contain single-stranded interruptions but not gaps, and are missing terminal sequences. These results are interpreted in terms of concatemer formation by end-to-end recombination between newly synthesised molecules whose terminal sequences are degraded in recombination-defective infections.