Five complementing recessive mutations that exhibit increased bacterial transposon Tn5 precise excision in yeast Saccharomyces cerevisiae were obtained by ethylmethanesulfonate treatment. One of these mutations (tex1) was submitted to extensive genetic analysis. tex1 is a recessive temperature-sensitive mutation resulting in a 20-100-fold increase in Tn5 excision. It also has increased frequencies of ochre mutation reversion, of forward mutation to canavanine resistance, and loss of chromosome III or its right arm. The possible mechanism of tex1 effects is discussed.

We have demonstrated that precise excision of bacterial transposon Tn5 can occur in the yeast, Saccharomyces cerevisiae. Tn5 insertions in the yeast gene LYS2 were generated by transposon mutagenesis made in Escherichia coli by means of a lambda::Tn5 vector. Nine insertions of Tn5 into the structural part of the yeast LYS2 gene situated in a shuttle episomal plasmid were selected. All the plasmids with a Tn5 insertion were used to transform yeast strains carrying a deletion of the entire LYS2 gene or a deletion of the part of LYS2 overlapping the point of insertion. All insertions inactivated the LYS2 gene and were able to revert with low (about 10(-8) frequencies to lysine prototrophy. Restriction analysis of revertant plasmids revealed them to be indistinguishable from the original plasmid without Tn5 insertion. DNA sequencing of the regions containing the points of insertions, made for two revertants, proved that Tn5 excision was completely precise.