Existence of two histone H3 variants in dicotyledonous plants and correlation between their acetylation and plant genome size

Histone H3 proteins were purified to near homogeneity from callus cultures of dicotyledonous plants alfalfa, soybean, Arabidopsis, carrot and tobacco to determine the number of histone H3 variants. In every species two histone H3 variants were identified by gradient gel electrophoresis and reversed-phase chromatography. They were named H3.1 and H3.2 in order of increasing mobility in acid-urea-Triton gels. Co-electrophoresis of histone H3.2 proteins of all species in this gel system and HPLC co-chromatography suggest that all histone H3.2 variants have a primary protein sequence identical to alfalfa H3.2. Two distinct H3.1 variant forms were identified, represented by alfalfa and Arabidopsis H3.1 proteins which differ only at residue 90. Soybean H3.1 resembles H3.1 of alfalfa. Carrot and tobacco H3.1 appear identical to the Arabidopsis H3.1 histone variant. All H3 proteins were acetylated to multiple levels and in each plant the histone H3.2 forms were more highly acetylated. An inverse relationship was observed between plant genome size and the relative abundance of histone variant H3.2 and also with the level of acetylation of both histone H3 variants. This correlation matches the general tendency that in plants with smaller genomes a larger fraction of the genome is transcriptionally active.