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A probabilistic model for genetic recombination of nonreplicating λ–phage DNA, stimulated by “mismatch repair” of UV photoproducts
Author(s) -
Hays John B.,
Hays Judith G.
Publication year - 1991
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360311312
Subject(s) - recombination , dna , recombinant dna , escherichia coli , bacteriophage , lysogenic cycle , homologous recombination , dna repair , dna replication , biophysics , chemistry , genetics , microbiology and biotechnology , biology , physics , gene
Genetic recombination of nonreplicating phage λ‐DNA, during infection of homoimmune lysogenic bacteria, was previously observed to be dramatically stimulated by prior uv irradiation of the phages, even when the Escherichia coli hosts lacked the major uv‐photo‐product excision‐repair system (UvrABC). UvrABC‐independent recombination of circular phage molecules depends on host MutHLS functions and on undermethylation of adenines at GATC sites in the phage DNA, and thus appears to be the result of “mismatch repair” of uv photoproducts. Recombinant frequencies pass through a relatively sharp maximum at 20 J/m 2 and decrease at higher doses, whereas most plausible models for the process predict monotonic increases with dose, or a plateau at high uv doses. A uv‐dose‐dependent loss of biological activity (restriction) of all intracellular phage DNA was also observed previously. In order to provide a framework for testing possible explanations for the unusual recombinant‐frequency vs uv‐dose curve, a statistical model was constructed. This model includes probability terms for all possible one‐exchange and two‐exchange recombination processes, and incorporates the assumption that dimer recombinants are more susceptible to restriction than monomer parents (or recombinants), because of their larger target size. By adjustment of model parameters, particularly ϵ, the efficiency per photoproduct of initiation of a recombinational exchange, a theoretical dose–response curve that agreed well with experiment was obtained. The best fit corresponded to ϵ = 0.035, close to the previously observed restriction efficiency of 0.053. In the calculations, the value for h 0 , the average length of heteroduplex DNA, was taken to be 0.5 λ units, i.e., about 25 kilobase pairs. This estimate for h 0 was obtained here by analysis of the density distributions of the progeny of crosses between nonreplicating density‐labeled λ‐phage chromosomes, published by others [M. S. Fox, C. S. Dudney and E. J. Sodergren (1979) Cold Spring Harbor Symposium on Quantitative Biology , Vol. 43, pp. 999–1007].