High mobility group proteins: abundance, turnover, and relationship to transcriptionally active chromatin

We have measured the abundance of high mobility group (HMG) proteins 14 and 17 in HeLa cell chromatin and their fractionation with respect to transcriptionally active sequences. HMG protein 17 constitutes 10-20% of the mass of an individual core histone; HMG 14 is approximately one-tenth the mass of HMG 17. The enrichment of HMG proteins, relative to bulk chromatin, is less than 2-fold in the chromatin fraction enriched 6-fold in active sequences. The digestion characteristics of HMG nucleosomes indicate that they are interspersed with H1 nucleosomes and other monomer species. The HMG monomers are quite resistant to degradation by micrococcal nuclease and can be resolved as distinct nucleoprotein entities after trimming of the DNA to core length. Turnover measurements showed that HMG proteins 14 and 17 are stable for at least 24 h. When nucleosome monomers are reconstituted with a 0.35 M NaCl nuclear protein extract, each nucleosome subtype can be reconstituted; however, this is a function of both the amount of extract added and the DNA length of the nucleosomes. When the kinetics of reconstitution of bulk vs. coding sequences were measured with cDNA, there was no significant enrichment of active sequences in the HMG-containing mononucleosomes of HeLa cells at any ratio of extract to monomer employed. In Friend cells, the abundance of sequences among mononucleosome species was the same for the transcribed beta-major globin gene, a transcriptionally inactive embryonic globin, and an inactive immunoglobulin gene. There was little correlation of HMG content with transcriptionally active chromatin, either native or reconstituted.