Studies on the biological role of DNA methylation: V. The pattern ofE.coliDNA methylation

Abstract

The distribution of the methlatable sites GATC and $CCTAGG$ was studied by analyzing the molecular average size of restriction fragments of E. coli DNA. Both sites were found to be randomly distributed, reflecting a random patterm of methylation. The methylation pattern of specific sequences such as the origin of replicationand rRNA genes has been studied in wild type E. coli and a metholation deficient (dam⁻ dcm⁻)mutant. These sequences were found to be methlated in wild type cells and unmethylated in the mutant indicating that there is no effect of the state of methylation of these sequences on their expression. Analysis of the state of methlation of GATC sistes in newly replicating DNA using the restriction enzyme Dpn I (cleaves only when both strands are methylated) reavealed no detectable hemimethlated Dna suggesting that methylation occurs at the replication fork. Taking together the results presented here and previously published ata (5), we arrive at the conclusion that the most likely function of E. coli ENA methylations is probably in preventing nuclease activity.