Abstract
Rat lung tissue was labeled in vitro with [3H]leucine, and nuclei were prepared. They were digested with deoxyribonuclease II, and 4 subfractions were isolated after differential centrifugation: MgCl2-soluble (active) and MgCl2-insoluble (inactive) chromatin, nuclear matrix sediment and matrix extract, using 2 M NaCl. The matrix extract fraction was enriched in radioactive DNA after a short pulse of [3H]thymidine. The labeling kinetics of histones were similar in each subfraction suggesting that histones were not preferentially incorporated onto nascent DNA. Nonhistones isolated from the subfractions, except for the matrix sediment fraction, followed closely, similar incorporation kinetics with [3H]leucine. The matrix sediment fraction was 3 times more actively labeled than nonhistones of the other fractions and displayed a unique protein composition, suggesting distinct functional properties.