Next, they purified the Swi5-Sfr1 helper proteins from cells to identify the precise regions within Module 1 that attach to Rad51. Then, by mutating the protein sequence, they were able to modify these regions in a way that prevents Swi5-Sfr1 from attaching to Rad51. Surprisingly, they found that although the mutated helper proteins could not switch on Rad51 in a test tube, yeast cells with these mutations were still able to repair their DNA without problems. This led the team to speculate that another group of helper proteins, which are present in the cell but absent in the test tube, was rescuing the DNA repair process. Previous genetic studies have shown that there are two HR sub-pathways in yeast — one that depends on Swi5-Sfr1 and another that relies on molecules called Rad51 paralogs. To test whether it was this other HR pathway that was rescuing DNA repair, the team used yeast that lacked the Rad51 paralogs. The results were striking: in yeast with mutant Swi5-Sfr1 and no Rad51 paralogs, the DNA damage was much more severe. This suggests that the damaging effects of mutations to the Swi5-Sfr1 helper complex are suppressed by a second group of helper proteins.
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