Interaction between short repeats may be a source of genomic rearrangements and deletions. We investigated possible interactions between short (9 base pairs) direct repeats in yeast by using our previously described system for analyzing bacterial transposon TnS excision in yeast. Mutations of either POL3 or POLI, the proposed structural genes for polymerases 8 and a, respectively, yield high levels of excision at semipermissive temperatures. poi2 (correspond to polymerase E) and po12 pol3 double mutants do not exhibit enhanced excision. A majority of excision events involve direct repeats and are precise; the remaining imprecise excisions occur within or in the vicinity of the repeats. The three DNA repair pathways identified by radi, rad6 and radl8, radSO and rad52 mutations were examined for their possible role in TnS excision; no enhancement was observed in mutants. However, the pol3-stimulated TnS excision was reduced in rad52 and radSO mutants. This suggests the potential for interaction between the systems for DNA double-strand break/recombi- polymerases 8 and a are responsible for lagging-strand synthesis and that polymerase e is responsible for leading-strand synthesis, we suggest that TnS excision is stimulated under conditions of altered lagging-strand synthesis, possibly due to extended opportunities for single-strand interactions between the inverted insertion sequence 1550 repeats of TnS.Short repeats of DNA sequence are a source of genomic rearrangements, duplications, and deletions. In bacteria, precise excision occurs between short, 9-base-pair (bp) direct repeats of a transposon target sequence. Excision is stimulated by the long, internal, inverted insertion sequences (IS). Molecular and genetic studies suggest that such deletion may result from "illegitimate" recombination between short repeats. The most likely mechanism is the "slippage" of a replicating strand (1-3). The same mechanism was proposed for formation of deletions between short direct repeats (4). TnS and TnOO excision in Escherichia coli is influenced by genes involved in recombination (recB, recC), mismatch correction (dam, mutH, mutL, mutS, uvrD), single-stranded DNA-binding protein (ssb), and the proofreading exonuclease subunit of DNA polymerase III (dnaQ) (5,6). Precise excision of a small, 50-bp palindromic TnJO remnant is increased in polA (DNA polymerase I) mutants (6).Based on results from bacteria, transposon excision may provide a means for studying various DNA metabolic processes, including DNA repair, recombination, and replication. We therefore introduced TnM into the yeast Saccharomyces cerevisiae and examined excision from the L YS2 gene (7). The low excision rate (10-9) was increased 100-fold in tex mutants (8). These and several other yeast DNA repair and metabolism mutants have been examined in order to understand the TnS excision process. Mutations in the structural genes for two of three yeast DNA polymerases (polymerase a and 5, but not polymerase E) greatly enhance the excision frequency. These obse...

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.