Studies on the nucleotide arrangement in deoxyribonucleic acids: X. Frequency and composition of pyrimidine isostichs in microbial deoxyribonucleic acids and in the DNA of E. coli phage T 3

Abstract

DNA preparations from Bacillus cereus, Escherichia coli, Salmonella typhimurium and typhosa, Serratia marcescens, and Rhodopseudomonas spheroides and also from bacteriophage T 3 have been studied with respect to the distribution and the composition of the pyrimidine isostichs released by mild acid hydrolysis. Two analytical procedures were employed in these investigations, namely the direct two-dimensional chromatography of the hydrolysate and the fractionation of the fragments on DEAE-cellulose. These DNA specimens showed several common characteristics: (a) The frequencies of the total pyrimidine isostichs of length 1 to 8 are similar in all preparations and correspond to the distribution predicted for a random polynucleotide. (b) The relative frequencies of individual oligonucleotide runs liberated by the various AT- and GC-types of DNA vary greatly from each other and deviate from the calculated random frequencies. (c) There exists a disparity in the relative contribution of cytosine and thymine to the isostichs of length 1 to 5: the shorter sequences are relatively richer in the first pyrimidine, the longer ones in the second. (d) Polycytidylic acid runs are comparatively very rare, whereas the frequency of polythymidylic acid sequences often exceeds random predictions. (e) DNA from genetically related species ( E. coli, S. typhimurium and S. typhosa) exhibit very similar patterns of pyrimidine arrangement. The DNA of phage T 3 resembles, also in the frequency of many of its individual pyrimidine sequences, a randomly constructed polynucleotide. Though it has almost the same base composition as the DNA of its host species, E. coli, it differs from the latter in sequence characteristics.