Methylation-induced blocks to in vitro DNA replication.

Single-stranded primed M13mp2 templates and double-stranded templates were treated with either dimethyl sulfate (DMS) or N-methyl-N'-nitro-N-nitrosoguanidine and used for DNA synthesis in vitro. Methylation inhibits the ability of the molecules to serve as templates. When either E. coli DNA polymerase I or AMV reverse transcriptase were used as polymerases, DNA synthesis terminated one nucleotide 3' to the site of adenine residues in the template. Heating of the templates resulted in the appearance of additional termination bands one nucleotide before the site of G's in the template. We assume that methylated A's but not methylated G's are blocks to in vitro DNA synthesis and that heating converts a portion of the sites of methylated G to AP sites which are blocks to synthesis.