Tunicate muscle actin genes. Structure and organization as a gene cluster.

We have isolated and determined the complete nucleotide sequences of two genes, HrMA4a and HrMA2, which encode the same muscle actin protein of the tunicate Halocynthia roretzi. HrMA4a and HrMA2 contain three exons, and the genes have intron-exon splice junctions at the same positions. The 5' flanking region of HrMA4a gene contains several potential regulatory elements. A TATA box is located at -30 and a CArG box found in regulatory region of vertebrate muscle-specific genes is located at -116. Seven E-box consensus sequences (CANNTG) known as binding sites for vertebrate myogenic determination factors are found within a 500 base-pair portion of the 5' flanking region of HrMA4a gene. HrMA4a and HrMA2 are separated by 1600 bases in genomic DNA and transcribed in the same direction. In addition to these genes, we have identified three other actin genes encoding muscle-type actins. All five actin genes are located in a 30 x 10(3) base-pair region of the genome and aligned in the same direction. This is the first report of a cluster of "vertebrate-type" muscle actin genes. The consensus sequences of 5' flanking region are conserved among these five genes, suggesting that the expression of the genes is controlled coordinately. This may be advantageous for the accumulation of considerable amounts of actin proteins in rapidly developing embryos of this animal.