Strand interruptions confer strand preference during intracellular correction of a plasmid-borne mismatch in Saccharomyces cerevisiae

Site-directed mutagenesis was used to construct yeast centromere plasmids i n which a strand nick or gap could be placed 5' or 3', on either strand, to a reporter gene (SUP4-o) carrying defined base mismatches. The plasmids we re then transformed into yeast cells and the direction and efficiency of mi smatch repair were assayed by scoring colouring of the transformant colonie s. Strands that were nicked were consistently corrected more often than int act strands, but the effect was very small. However, placement of a small g ap at the same positions as the nicks resulted in a marked increase in sele ction for the gapped strand and an enhanced efficiency of mismatch repair. Both the preference for the gapped strand and correction of the mismatch we re offset by deletion of the mismatch repair gene PMS1. Together, the resul ts suggest that strand interruptions can direct intracellular mismatch corr ection of plasmid-borne base mispairs in yeast.