Histone-histone interactions in a lower eukaryote, Tetrahymena thermophila

Abstract

The 6 pairwise interactions of Tetrahymena macronuclear histones H2A (formerly HX), H2B, H3 and H4 were studied using the techniques of circular dichroism and continuous variation. Parallel experiments were performed with calf thymus histones as controls, and 12 possible interspecies pairs were examined. The behavior of 3 of the ciliate histones, H2A, H2B and H3, is virtually identical with that of their vertebrate counterparts. Tetrahymena H4 exhibits a pattern of interactions identical with that of calf thymus H4, but displays significant quantitative differences in the strength of those interactions. The possibility that these differences may be artifactual is not entirely eliminated but they may reflect the unique primary structure of Tetrahymena H4. The pattern of histone-histone interactions defined for plants, animals and fungi includes the protists. A strong evolutionary conservatism of this pattern is implied in spite of the electrophoretic and compositional differences which exist between homologous histones of different organisms. Histone HX should be redefined as Tetrahymena H2A.