Hemimethylation prevents DNA replication in E. coli

The DNA adenine methylase of E. coli methylates adenines at GATC sequences. Strains deficient in this methylase are transformed poorly by methylated plasmids that depend on either the pBR322 or the chromosomal origins for replication. We show here that hemimethylated plasmids also transform dam − bacteria poorly but that unmethylated plasmids transform them at high frequencies. Hemimethylated daughter molecules accumulate after the transformation of dam − strains by fully methylated plasmids, suggesting that hemimethylation prevents DNA replication. We also show that plasmids purified from dam + bacteria are hemimethylated at certain sites. These results can explain why newly formed daughter molecules are not substrates for an immediate reinitiation of DNA replication in wild-type E. coli.