CAN TETRAPLEX RECOMBINATION MODELS EXPLAIN OBSERVATIONS IN INDUCED MITOTIC GENE CONVERSION

Induced mitotic gene conversion studies on the CYC1 gene of yeast have shown that the actual base pair changes, the types of changes (base s ubstitution, deletion or addition) and the distances between mutations all affect gene conversion yields. In crosses between mutations less than four bases apart, gene conversion rates are as low as back mutati on rates. The same mutants crossed to alleles more than five bases awa y may recombine 50-fold more. In crosses between mutations five or mor e base pairs apart, recombination rates varying by up to ten-fold are observed when comparing mutations at the same codon sites. The actual mutations in crosses affect recombination rates at these distances. Th e data rules out models in which mutants are repaired independently. M odels with large gaps at the initiation site are ruled out if the muta nts are within the gap. Recombination models are favoured in which bot h mutations can interact at a distance to affect the probability of re combination; such interactions may reflect the geometry of the recombi national junctions. The specific interactions proposed are that the ac tual mutations, and residual mismatches arising on excision resynthesi s, affect both the further migration of the recombinational junction, and the probability that excision-repair will detect and correct resid ual mismatches. Junction models in which interactions are expected inc lude those composed of base tetraplexes. The data is interpreted in te rms of these models. Meiotic recombination data is consistent with the se models. (C) 1995 Academic Press Limited