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Amino acid analysis of proteins and tightly bound phosphopeptides in calf thymus DNA: indications of a novel regulatory mechanism.

Purification of DNA prepared from calf thymus nuclei, which were isolated at temperatures of -10 degrees to -15 degrees C, termed N-DNA, reduced the protein content from 17% to 0.7%. Amino acid analysis of the crude N-DNA revealed that the protein material removed in this process contained lys and phe, whereas in purified N-DNA neither of these amino acids was present. The crude N-DNA had a lower total Pser molar ratio (5.9) than that of the purified N-DNA (22.6). The purification also effected a large decrease of molecular weight, from 170 to 15 million daltons, roughly corresponding to the size of replicons. The amino acid composition of the N-DNA was compared with those of the two PP fractions, a) the PPs released into the dialysates, and b) the core material bound in the DNA, probably covalently, after reaction of N-DNA with EDTA and subsequent dialysis. N-DNA had an intermediate total Pser molar ratio between that of a) (35.3) and b) (11.9). Analysis of a standard preparation of DNA (S-DNA), which involved initial rupture of nuclei in the presence of cytoplasmic components, gave elevated values of asp and glu, a total Pser average value of 8.7 before, and 9.0, after the purification, and again phe and lys in the crude DNA.