Differences in the mode of iodination of H2a variants in chromatin

Stably DNA-bound chromosomal proteins

DNA accomplishes its biological function in a complex with nuclear proteins. A minor protein fraction has been found in chromatin which could not be dissociated from DNA by reagents abolishing non-covalent type of interactions. The controversy surrounding the nature of the protein moiety and the nature of the bond linking the two components on the one hand, and the fact pointing to its evolutionary conservatism and metabolic stability on the other, make it necessary to critically evaluate the data in view of the possible biological function for such proteins.

Histone H1 in the centromeric heterochromatin of Glyptotendipes barbipes larval polytene chromosomes

Immunofluorescent analysis with antibodies against histone H1 failed to detect H1 in the centromeric heterochromatin blocks of the polytene chromosomes of Glyptotendipes barbipes larvae. Centromeric regions were dissected microsurgically and acid-extracted. Electrophoresis in SDS and acid-urea gels revealed a band comigrating with H1 of calf thymus and of Gl. barbipes salivary gland nuclei. ELISA dot assay of the extracted material gave a positive reaction with anti-H1 monoclonal antibodies and with anti-H1 affinity-purified polyclonal antibodies. This shows that the centromeric heterochromatin contains histone H1 but packed in a way which prevents the H1 antigenic determinants from reacting in situ with the specific antibodies.

An evolutionarily conserved protein fraction stably linked to DNA

Chromatins from four evolutionarily remote species (insect, fish, amphibian and bird) were isolated, high-salt-extracted and extensively deproteinized to remove noncovalently associated proteins. A protein fraction resisting the extraction procedures was found firmly linked to DNA in all four chromatins. Two-dimensional tryptic peptide mapping revealed a remarkable evolutionary conservativeness of this protein component, suggesting an indispensable function for it in the nucleus.

Metabolic behaviour of a stable DNA-protein complex

1. A stable DNA-protein complex resisting all treatments dissociating noncovalently associated proteins was isolated from mouse erythroleukemia cells. 2. Two-dimensional tryptic peptide mapping of this DNA-linked protein component revealed a remarkable similarity to the maps of the corresponding proteins from other mammalian chromatins. 3. Labelling experiments showed that the protein component was metabolically stable.