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The E coli dam (DNA adenine methylase) enxyne is known to nethylate the sequence GATC. A general nethod for cloning sequence-speoific DNA methylase genet was used to isolate the dam gene on a 1.14 kb fragment, inserted in the plasmid vector pBK322. Subsequent restriction mapping and subcloning experiments established a set of approximate boundaries of the gene. The nucleotide sequence of the dju gene was determined, and analysis of that sequence revealed a unique open reading frame which corresponded in length to that necessary to oode for a protein the sixe of dam. Amino acid conposition derived froa this sequence corresponds closely to the amino acid composition of the purified dam protein. Enxynatic and DNA:DNA hybridixation methods were used to investigate the possible presence of dasi genes in a variety of prokaryotlc organisms.