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It has been known for several years that deoxyribonucleic acid may normally contain 5-methylcytosine (Wyatt, 1951), 5-hydroxymethylcytosine (Wyatt & Cohen, 1953), and 6-methylaminopurine (Dunn & Smith, 1955, 1958a) in addition to adenine, guanine, cytosine and thymine. The first evidence of such additional bases naturally present in ribonucleic acid is the recent isolation of an unidentified nucleotide from the ribonucleic acid of yeast (Cohn, 1957; Davis & Allen, 1957). During studies on the incorporation of unnatural bases, Dr J. D. Smith and one of us (D. B. D.) each noted that digests of ribonucleic acid from E8cherichia coli contained a compound with spectral and chromatographic characteristics which suggested that it was the riboside of thymine (J. D. Smith, unpublished work; Dunn, 1957). The present work has not only confirmed this finding but has indicated that three methylated adenine bases, 2-methyladenine, 6methylaminopurine and 6-dinethylaminopurine, as well as thymine, are naturally present in small amounts in ribonucleic acid from a variety of sources. None of these compounds has been described previously as a component of ribonucleic acid, but all four are known to occur naturally as nucleosides or nucleotides, since thymine and 6methylaminopurine occur in deoxyribonucleic acid (Kossel & Neumann, 1893; Dunn & Smith, 1958a), 2-methyladenine in the vitamin B12-like Factor A extracted from E. coli and other sources (Dion, Calkins & Pfiffner, 1954; Brown, Cain, Gant, Parker & Smith, 1955) and 6-dimethylaminopurine in puromycin formed by the actinomycete Streptomyces alboniger (Waller, Fryth, Hutchings & Williams, 1953). The presence of 6-methylaminopurine in ribonucleic acid from yeast has also been reported by Adler, Weissmann & Gutman (1958). This paper describes the isolation of the ribosides and ribotides of these compounds from several ribonucleic acids and their identification by chromatographic, electrophoretic and spectroscopic methods. Preliminary results of this work have been reported (Littlefield & Dunn, 1958a, b).

The occurrence and distribution of thymine and three methylated-adenine bases in ribonucleic acids from several sources