Studies on the nucleotide arrangement in deoxyribonucleic acids VI. Pyrimidine nucleotide clusters: Frequency and distribution in several species of the AT-type

Abstract

The pyrimidine oligonucleotide clusters released by the hydrolysis of DNA specimens from calf thymus, rye germ, and wheat germ have been studied. Sequences of the general structure (pyrimidine deoxyriboside) n (phosphorric acid residue) n+1 with n of 1–8, have been separated quantitatively and their distribution in the three DNA preparations has been compared. Clusters of 9 or more pyrimidines in a row account for 6–8 % of the pyrimidines. In all cases, the results deviate considerably from what would be expected from a random arrangement of clusters. Almost all oligonucleotide units, comprised in the fractions with n of 1–4, have been separated, characterized, and estimated. Whereas the totol abundance of the various oligonucleotide classes was rather similar in the three DNA specimens, which were all of the AT-type, calf-thymus DNA differed widely from the specimens from rye and wheat germ in the amounts of individual oligonucleotides isolated. Some pyrimidine oligonucleotides which could have been expected were not encountered, conspicuously those consisting of 4 or 5 cytosine units in a row, whereas the corresponding thymine oligonucleotides were observed invariably. Previous observations showing that in the plant nucleic acid 5-methylcytosine and cytosine do not share the same pattern of arrangement within the polymer have been confirmed. The bearing of some of the findings on the general problem of DNA structure is considered.