Histone variants specific to the transcriptionally active, amitotically dividing macronucleus of the unicellular eucaryote, Tetrahymena thermophila

Two-dimensional gel electrophoresis (triton-acid-urea followed by SDS) has been used to resolve two previously uncharacterized, quantitatively minor histone variants in acid extracts from macronuclei of Tetrahymena thermophila. Utilizing techniques which allow characterization of these variants without purifying them in significant quantities, we identify one protein as a subtype of H3. The other protein is a moderately lysine-rich histone whose tryptic peptide map differs from that of both H2A and H2B. However, its pattern of secondary modifications, its detergent-binding properties and its methionineless nature all suggest that it is more like H2A than any other histone. Both variants are associated with nucleosomes derived from macronuclei. Thus primary sequence variants of the inner histones, presumably indicative of nucleosome heterogeneity, exist in a lower eucaryote, in an amitotic nucleus, and within the nucleus of a clonally propagated organism. Evidence is presented that these newly described minor variants are absent in micronuclei, suggesting that they play an important role in the structural and functional differentiation of macronuclear chromatin.