COMMON FEATURES OF ANALOGOUS REPLACEMENT HISTONE H3 GENES IN ANIMALS AND PLANTS

Phylogenetic analysis of histone H3 protein sequences demonstrates the independent origin of the replacement histone H3 genes in animals and in plants, Multiple introns in the replacement histone H3 genes of an imals in a pattern distinct from that in plant replacement H3 genes su pports this conclusion. It is suggested that replacement H3 genes aros e at the same time that, independently, multicellular forms of animals and of plants evolved. Judged by the degree of invariant and function ally constrained amino acid positions, histones H3 and H4, which form together the tetramer kernel of the nucleosome, have co-evolved with e qual rates of sequence divergence. Residues 31 and 87 in histone H3 ar e the only residues that consistently changed across each gene duplica tion event that created functional replacement histone H3 variant form s. Once changed, these residues have remained invariant across diverge nt speciation. This suggests that they are required to allow replaceme nt histone H3 to participate in the assembly of nucleosomes in non-S-p hase cells. The abundant occurrence of polypyrimidine sequences in the introns of all replacement H3 genes, and the replacement of an intron by a polypyrimidine motif upstream of the alfalfa replacement H3 gene , suggests a function. It is speculated that they may contribute to th e characteristic cell-cycle-independent pattern of replacement histone H3 genes by binding nucleosome-excluding proteins.