Genetic exchanges induced by structural damage in nonreplicating phage lambda DNA.

Abstract

Genetic recombination between irradiated lambda phage and the unirradiated lambda prophage in homoimmune lysogens has been studied under conditions in which phage DNA replication and repair were controlled. The lambda phage were exposed to one of three treatments before infecting the lysogens: (a) 254-nm light, which produces pyrimidine dimers and other photoproducts; (b) 313-nm light with acetopheneone D, which produces thymine dimers and a different spectrum of other photoproducts; (c) 360-nm light with trimethylpsoralen, which produces monoadducts and cross-links. With both replication and excision-repair of the damaged phage DNA blocked, treatment b (acetophenone D) caused no significant increase in recombination, indicating that thymine dimers do not cause recombination if the DNA in which they are contained is not replicated. Treatment a (254 nm), producing the same total number of pyrimidine dimers, caused a marked increase in recombination. This indicates that photoproducts other than pyrimidine dimers produced by 254-nm light can cause recombination in the absence of replication. Treatment c (psoralen) caused a marked increase in recombination in wild type but not in uvrA and uvrB mutants. The frequency of recombination in two-factor crosses varied with marker separation in such a way as to suggest that cross-links can act over distances of at least 5% of the lambda genome to cause exchanges between pairs of relatively closely spaced markers. The psoralen photo cross-links and monoadducts initiate recombination only following the action of excision enzymes, which appears to release one arm of each cross-link, producing a gap with free strand ends. It may be these strand ends which induce recombination. The action at a distance of 5% of the lambda genome may reflect heteroduplex formation and the subsequent reduction to homozygosity of mismatched base pairs at genetic markers. Recombination between closely spaced markers in the P gene is reduced in strains carrying polA.