Methylated bases are found in deoxyribonucleic acid (DNA) of bacteria and mammalian cells, but the biological role of such bases is unknown. It is proposed to use Escherichia coli and its viruses as a model system to determine the function of 6-methyladenine (6-meA) at the molecular level. The applicant, in collaboration with Dr. N. R. Morris of Rutgers Medical School, has isolated strains with reduced amounts of 6-meA in DNA (dam mutants). Studies with these mutants indicated that they (1) contained single strand breaks in DNA, (2) were inviable if unable to repair these breaks in DNA, (3) were sensitive to various carcinogenic and mutagenic agents, (4) had an abnormal morphology and 5) had a high frequency of mutability. These results, together with those of other investigators, suggest that 6-meA is essential for DNA replication, DNA repair and DNA recombination. The objectives of the present proposal will be (1) to isolate a mutant of E. coli devoid of 6-meA in DNA, (2) to determine which step in phage fd reproduction is blocked in dam repair deficient mutants, (3) to purify the dam DNA adenine methylase, (4) to purify the hypothetical endonuclease that degrades 6-meA deficient DNA, (5) to investigate further the hyper-recombination phenotype of dam mutants, and (6) to isolate RNA methylase deficient mutants.