The Escherichia coli dam adenine-N6 methyltransferase modifies DNA at GATC sequences. It is involved in post-replicative mismatch repair, control of DNA replication and gene regulation. We show that E. coli dam acts as a functional monomer and methylates only one strand of the DNA in each binding event. The preferred way of ternary complex assembly is that the enzyme first binds to DNA and then to S-adenosylmethionine. The enzyme methylates an oligonucleotide containing two dam sites and a 879 bp PCR product with four sites in a fully processive reaction. On lambda-DNA comprising 48,502 bp and 116 dam sites, E. coli dam scans 3000 dam sites per binding event in a random walk, that on average leads to a processive methylation of 55 sites. Processive methylation of DNA considerably accelerates DNA methylation. The highly processive mechanism of E. coli dam could explain why small amounts of E. coli dam are able to maintain the methylation state of dam sites during DNA replication. Furthermore, our data support the general rule that solitary DNA methyltransferase modify DNA processively whereas methyltransferases belonging to a restriction-modification system show a distributive mechanism, because processive methylation of DNA would interfere with the biological function of restriction-modification systems.

(c) 2002 Elsevier Science Ltd.