Different frameshift mutation spectra in non-repetitive DNA of MutSα- and MutLα-deficient fission yeast cells

Abstract

A frameshift reversion assay has been established for Schizosaccharomyces pombe, which allows detection of deletions and insertions of nucleotides in a non-repetitive DNA sequence. Compared to wild type, frameshift mutation rates were increased in the mismatch repair (MMR) mutants msh2, msh6, mlh1, and pms1, but not in a swi4 strain (defective in the Msh3 homologue). Rates were also elevated in the DNA nuclease-deficient strains rad2 (defective in the FEN-1 homologue) and exo1. In MutSα-deficient strains, msh2 and msh6, most of the reversions were 1 bp deletions. In contrast, mlh1 and pms1 mutants, defective in MutLα, accumulated significantly more 2 bp insertions, preferentially of the type CG to (CG) 2. Such duplications were less frequent in double mutants additionally defective in msh2, msh6, rad2, or exo1. Thus, accumulation of (CG) 2 in MutLα-deficient strains depends on the presence of MutSα, Rad2 and Exo1.