Nucleosome periodicity in HeLa cell chromatin as probed by micrococcal nuclease.

Abstract

When HeLa [human cervical cancer] cell nuclei were treated with micrococcal nuclease (nucleate 3-oligonucleotidohydrolase, EC 3.1.4.7), lysed and centrifuged, the supernatant from early digests contained 2 predominant classes of polynucleosomes of repeat size 8N and 16N. With increasing digestion time, the 16N polynucleosome appeared to be cleaved to the 8N species and finally to the basic subunit of chromatin. The size of the polynucleosomes was determined by DNA analysis and on polyacrylamide electrophoretic gels of native chromatin particles. The 16N polynucleosome appears to be a unique higher ordered structural component of HeLa cell chromatin. A recent report, showing that the nuclear protein-modifying enzyme poly(ADP-ribose) polymerase increases in specific activity progressively with increasing nucleosome repeat size up to 8-10N, was extended in the present study. Activity was also elevated in the polynucleosomes of the 16N structure preferentially cleaved by micrococcal nuclease, although specific activity of the enzyme was highest in octanucleosomes. Acceptors for poly(ADP-ribose) were also determined in these particles.