Expression of the Escherichia coli dam methylase in Saccharomyces cerevisiae: effect of in vivo adenine methylation on genetic recombination and mutation.

The Escherichia coli DNA adenine methylase (dam) gene has been introduced into Saccharomyces cerevisiae on a yeast-E. coli shuttle vector. Sau3AI, MboI, and DpnI restriction enzyme digests and Southern hybridization analysis indicated that the dam gene is expressed in yeast cells and methylates GATC sequences. Analysis of digests of total genomic DNA indicated that some GATC sites are not sensitive to methylation. The failure to methylate may reflect an inaccessibility to the methylase due to chromosome structure. The effects of this in vivo methylation on the processes of recombination and mutation in mitotic cells were determined. A small but definite general increase was found in the frequency of mitotic recombination. A similar increase was observed for reversion of some auxotrophic markers; other markers demonstrated a small decrease in mutation frequency. The effects on mutation appear to be locus (or allele) specific. Recombination in meiotic cells was measured and was not detectably altered by the presence of 6-methyladenine in GATC sequences.