Cross-Complexing Pattern of Plant Histonest

Pea histones H2a, H2b, H3, and H 4 have been isolated and their interactions studied by fluorescence anisotropy, light scatter, and circular dichroism. Histones H3 and H 4 are almost identical in plants and animals, but plant histones H2a and H2b differ markedly from their mammalian counterparts. Pea H2b has a molecular weight approximately 20% greater than that of calf thymus H2b; the amino acid compositions of the two proteins are different. Calf thymus H2a exists as a single molecular weight species, while pea H2a exists as two species which differ by about 1500 daltons. The larger plant H2a is about 1% greater in molecular weight than T h e subunit structure of chromatin has been demonstrated by nuclease digestion and electron microscopy (Olins and Olins, 1973, 1974; Woodcock, 1973; Van Holde et al., 1974; Kornberg, 1974). A general picture has emerged in which two each of the histones H2a, H2b, H3, and H4 associate strongly with one another to form a globular histone core. The DNA is wrapped about the core (Pardon et al., 1975). The subunit structure of chromatin has now been found in a wide variety of organisms (Lohr and Van Holde, 1975; Griffith, 1975; McGhee and Engle, 1975; Gorovsky and Keevert, 1975; Jerzmanowski et al., 1976; Morris, 1976; Noll, 1976; Thomas and Furber, 1976; Nicolaieff et al., 1976). Two of the histones, H 3 and H4, have the most highly conserved primary structures of any proteins known (DeLange et al., 1969; Patthy et al., 1973). The classic comparison between calf thymus H 4 and pea H 4 demonstrated (DeLange et al., 1969) that these differ by only two conservative replacements. Although we know the sequence of H 3 and H4 from widely divergent species (DeLange et al., 1969; Patthy et al., 1973), such knowledge is not yet available for H2a and H2b. Indeed, H2a and H2b must have changed to a much greater degree throughout evolution. Their molecular weights are much greater in plants than in animals (Sommer and Chalkley, 1974) and, as we point out in this paper, based on amino acid content, H2a and H2b have evolved perhaps more like cytochrome c than like the highly conserved H 3 and H 4 histones.