Genetic Exchanges Induced by Structural Damage in Nonreplicating Phage λ DNA
Genetic recombination between irradiated λ phage and the unirradiated λ prophage in homoimmune lysogens has been studied under conditions in which phage DNA replication and repair were controlled. The λ 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 λ 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 appear 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 λ 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.
KeywordsPyrimidine Dimer Mismatched Base Pair Thymine Dimer Marker Separation States Public Health Service Grant
Unable to display preview. Download preview PDF.