Limited action of micrococcal nuclease on trout testis nuclei generates two mononucleosome subsets enriched in transcribed DNA sequences.

Hybridization experiments show that DNA extracted from two distinct subsets of mononucleosomes (MNI and MN2) generated by a limited action of micrococcal nuclease on trout testis nuclei is enriched approximately 7-fold in sequences that are transcribed into cytoplasmic polyadenylated RNA in trout testis cells. Both subsets of mononucleosomes contain eight core histones, but MNI also possesses one or two molecules of a small, basic, high-mobility-group (HMG) protein H6 [Levy W., B., Connor, W. & Dixon, G. H. (1979) J. Biol. Chem. 254, 609-620], bound to a DNA fragment of 140 base pairs. In contrast, MN2 contains 1 molecule of H1 but no H6, and its DNA length is somewhat longer at 140-190 base pairs. The preferential release of these two subsets of mononucleosomes is correlated with the presence of a second larger HMG protein, HMG-T, in the linker regions flanking both types of mononucleosomes. The HMG-T-containing linker regions appear to be considerably more susceptible to attack by micrococcal nuclease than H1-containing linkers. Cross-reassociation reactions between the DNA from MN1 and MN2 subsets indicate that they share a significant extent of sequence overlap but also that each subset contains specific sequences that are absent in the other subset.