ADP ribosylation of rat liver lysine-rich histone in vitro.

Purified rat liver nuclei were incubated with [14C]-NAD+ and the various nuclear protein fractions were separated. Forty per cent of the total radioactivity incorporated was associated with the histone fraction. Of this, about 50% was extracted with H1, in 0.5 N perchloric acid. When crude H1 was purified and fractionated into five different subfractions by chromatography on Bio-Rex 70, it was found that all the H1 subfractions contained radioactivity. This radioactive material was identified as oligomers of adenosine diphosphate ribose (ADP-Rib) with an average chain length which corresponded to trimers. The extent of the modification was dependent on the concentration of NAD+. About 60% of the H1 molecules were modified with a concentration of 1 mM NAD+. The presence of these oligomers of ADP-Rib introduced a large degree of microheterogeneity to H1 as detected by electrophoresis in polyacrylamide gels containing 2.5 M urea and 0.9 N acetic acid. Bands of H1 with 10 to 20% less mobility than the unmodified H1 were present. Also, as a consequence of large content of ADP-Rib, the absorption maximum shifted from 275 to 259 nm. The half-life of the bond between the oligomers of ADP-Rib and H1 was about 3 min at 37 degrees C in the presence of 0.1 N NaOH, and 10 m1 were modified. The site of ADP ribosylation in the NH2-terminal half was localized in the tryptic peptide extending from the NH2-terminal end to lysine 15. The site of modification of the COOH-erminal half was localized in the tryptic peptide which contained the only glutamic acid residue in this fragment of H1...