SUBSTRATE LENGTH REQUIREMENTS FOR EFFICIENT MITOTIC RECOMBINATION IN SACCHAROMYCES-CEREVISIAE

An ectopic recombination system using ura3 heteroalleles varying in si ze from 80 to 960 bp has been used to examine the effect of substrate length on spontaneous mitotic recombination. The ura3 heteroalleles we re positioned either on nonhomologous chromosomes (heterochromosomal r epeats) or as direct or inverted repeats on the same chromosome (intra chromosomal repeats). While the intrachromosomal events occur at rates at least 2 orders of magnitude greater than the corresponding heteroc hromosomal events, the recombination rate for each type of repeat cons idered separately exhibits a linear dependence on substrate length. Th e linear relationships allow estimation of the corresponding minimal e fficient processing segments, which are approximately 250 bp regardles s of the relative positions of the repeats in the yeast genome. An exa mination of the distribution of recombination events into simple gene conversion versus crossover events indicates that reciprocal exchange is more sensitive to substrate size than is gene conversion.