Histone genes in three sea star species: cluster arrangement, transcriptional polarity, and analyses of the flanking regions of H3 and H4 genes.

The arrangement of core histone genes and their transcriptional polarity has been determined for three species of sea stars (Pisaster ochraceus, P. brevispinus, and Dermasterias imbricata) representing two orders which diverged over 500 million years ago. Each species has approximately 500 core histones cluster repeats per haploid genome. The close phylogenetic relationship between the Pisaster species is evident from the correspondence of restriction sites in the repeat element, identical arrangement of core histones, and high degree of sequence homology in both the coding and spacer regions of the H3 gene. The Dermasterias repeat has the same gene order and transcriptional polarity of core histones, but its restriction map is significantly different. Moreover, the Dermasterias H3 gene has the same amino acid sequence, but in comparison to Pisaster nucleotide sequences, shows a high level of silent substitutions. Analyses of the nucleotide sequence of the 5' and 3' regions surrounding the H3 gene from each species demonstrate the presence of appropriately spaced consensus and processing signal segments. The 3' spacer segment of the Dermasterias H4 gene contains an unusual, threefold tandemly repeated, 21-nucleotide, AT-rich sequence. No similar sequence is seen in the P. brevispinus H4 3' region, but these two species show a striking regularity of distribution of five different homologous oligomers in the 3' spacer.