Evolution of cis-acting elements in 5' flanking regions of vertebrate actin genes.

Regulation of the vertebrate actin multigene family involves the recognition of various regulatory sequences (cis-acting elements) that specify the distinct tissue type and developmental program of expression for each actin paralogue, which implies that the distribution of cis-acting elements may be unique for each paralogue gene. To elucidate the evolution of these unique distribution patterns, we improved a method to scan for cis-acting elements in the 5' flanking regulatory region of genes and used it to analyze five cis-acting elements (SRE, MyoD binding site, Elk-1 binding site, positive and negative YY1 binding sites) of six actin paralogue genes (beta and gamma cytoplasmic actins, alpha and gamma smooth muscle actins, and alpha skeletal and alpha cardiac actins) among various vertebrates. It was shown that although an element(s) may exist in all paralogue genes of the same species, its numbers, compositions, and distribution patterns or even sequences vary remarkably among paralogues, which contributes to their different tissue- and developmental-specific expression. However, each pair of coexpressed paralogues has some certain similarity in distribution patterns. Furthermore, among various orthologues of actin genes derived from diverse vertebrates, the sequences, numbers, and distribution patterns of these cis-acting elements are highly conserved or even identical in the long run of phylogeny of vertebrates. Taken together, the results described above strongly indicate that not only the structures of actins but also their expression patterns are essential in both the phylogeny and the physiology of vertebrates. The distribution patterns of cis-acting elements of various actin genes can be regarded as indicators of both horizontal (paralogues) and vertical (orthologous) evolution of actins.