Characterization of the trp5-27 allele used to monitor drug-induced mitotic gene conversion in the Saccharomyces cerevisiae tester strain D7.

Mitotic gene conversions, among other recombinagenic events, can play an important role in the multistep process of carcinogenesis. The ability of chemicals to induce such gene conversions can easily be monitored in the Saccharomyces cerevisiae tester strain YHE2, a derivative of strain D7. For the detection of drug-induced gene conversions, two mutations in the TRP5 locus are used, trp5-12 and trp5-27. Here we report on the characterization of the stable allele trp5-27. Our analysis revealed two relevant mutations in trp5-27: (a) a transition C to T at position 121 after ATG that results in an amber stop codon and abolishes gene expression and (b) a transversion A to T at position 1555 that creates an ochre stop codon. Simultaneous amber and ochre suppression with the suppressors SUP3 and SUP11, respectively, was capable of relieving the tryptophan-requiring phenotype of strains carrying the trp5-27 allele. These findings have implications on the length of gene conversion tracts in conversion events between trp5-12 and trp5-27: conversion tracts can cover several kilobases, if the site of the mutation in trp5-12 lies outside of the positions mutated in trp5-27. Conversely, the maximal length is limited to 1435 bp, if the mutation in trp5-12 is located between the positions mutated in trp5-27.