Labelling of histones and nonhistones in lung nuclear matrix and chromatin fractions.

Rat lung tissue is labelled in vitro with [3H]leucine and nuclei are prepared. They are digested with deoxyribonuclease II and four subfractions are isolated after differential centrifugation: MgCl2-soluble (active) and MgCl2-insoluble (inactive) chromatin, nuclear matrix sediment and matrix extract using 2M NaCl. The matrix extract fraction is found to be enriched in radioactive DNA after a short pulse of [3H]thymidine. The labelling kinetics of histones are similar in each subfraction, suggesting that histones are not preferentially incorporated onto nascent DNA. Nonhistones isolated from the subfractions, except for the matrix sediment fraction, also follow closely similar incorporation kinetics with [3H]-leucine. The matrix sedimnent fraction is three times more actively labelled than nonhistones of the other fractions and displaying a unique protein composition, suggesting distinct functional properties.