DNA methylation pattern is determined by the intracellular level of the methylase.

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Abstract
Extrachromosomal plasmid DNA is transiently undermethylated in Escherichia coli during amplification in the presence of chloramphenicol. Undermethylation of phage .lambda. DNA was observed after thermal induction of a .lambda.cI857 lysogen while the integrated .lambda. phage DNA was fully methylated. These methylation pattern changes occur under conditions (extensive replication) in which the intracellular methylase level becomes limiting. In an E. coli strain that harbors a plasmid that carries the dam methylase gene and therefore overproduces dam methylase, there is no undermethylation of dam sites in either of the extrachromosomal DNA. The sites that are methylated by the mec methylase in both plasmid and .lambda. phage DNA were undermethylated in the dam overproducer also. Apparently the intracellular level of the E. coli methylase determines the DNA methylation pattern.