The E. coli recA gene was introduced into the pso4-1 mutant of S. cerevisiae and transformants were treated with 8-MOP+UVA and 254-nm UV light. The results showed that the recA gene increased the resistance to the toxic effect of 8-MOP+UVA and restored the frequency of reversion of the pso4-1 mutants after both treatments. The presence of the recA gene stimulated expression of the small subunit of the ribonucleotide reductase (Rnr2) in the pso4-1 mutants. Thus the E. coli recA gene is functional in yeast. Moreover, it was shown that the pso4-1 mutant is epistatic to pso1-1 and rad6-1, which belong to a mutagenic repair pathway. We propose here that the PSO4 gene has some role in the control of mutagenic repair in yeast.