Rooney Alejandro P, Piontkivska Helen, Nei Masatoshi
Department of Biological Sciences, Mississippi State University, P.O. Box GY, Mississippi State, MS 39762, USA.
Mol Biol Evol. 2002 Jan;19(1):68-75. doi: 10.1093/oxfordjournals.molbev.a003983.
In some species, histone gene clusters consist of tandem arrays of each type of histone gene, whereas in other species the genes may be clustered but not arranged in tandem. In certain species, however, histone genes are found scattered across several different chromosomes. This study examines the evolution of histone 3 (H3) genes that are not arranged in large clusters of tandem repeats. Although H3 amino acid sequences are highly conserved both within and between species, we found that the nucleotide sequence divergence at synonymous sites is high, indicating that purifying selection is the major force for maintaining H3 amino acid sequence homogeneity over long-term evolution. In cases where synonymous-site divergence was low, recent gene duplication appeared to be a better explanation than gene conversion. These results, and other observations on gene inactivation, organization, and phylogeny, indicated that these H3 genes evolve according to a birth-and-death process under strong purifying selection. Thus, we found little evidence to support previous claims that all H3 proteins, regardless of their genome organization, undergo concerted evolution. Further analyses of the structure of H3 proteins revealed that the histones of higher eukaryotes might have evolved from a replication-independent-like H3 gene.
在一些物种中，组蛋白基因簇由每种组蛋白基因的串联阵列组成，而在其他物种中，基因可能成簇但并非串联排列。然而，在某些物种中，组蛋白基因分散在几条不同的染色体上。本研究考察了未排列成串联重复大簇的组蛋白3（H3）基因的进化。尽管H3氨基酸序列在物种内部和物种之间都高度保守，但我们发现同义位点的核苷酸序列差异很大，这表明纯化选择是在长期进化过程中维持H3氨基酸序列同质性的主要力量。在同义位点差异较低的情况下，近期的基因复制似乎比基因转换更能解释这种现象。这些结果以及关于基因失活、组织和系统发育的其他观察结果表明，这些H3基因在强烈的纯化选择下按照生死过程进化。因此，我们几乎没有证据支持之前的说法，即所有H3蛋白，无论其基因组组织如何，都经历协同进化。对H3蛋白结构的进一步分析表明，高等真核生物的组蛋白可能是从类似不依赖复制的H3基因进化而来的。
