Coexpression and promoter function in two muscle actin gene complexes of different structural organization in the ascidian Halocynthia roretzi.

During embryogenesis of the ascidian Halocynthia roretzi, 42 unicellular striated muscle cells are formed in the tail of the tadpole larva. Isolation of cDNA clones demonstrated that multiple genes for larval muscle actin are expressed in this process. Among them, at least five muscle actin genes (HrMA2, HrMA4a, HrMA4b, HrMA5, and HrMA6) form a cluster (HrMA2/4 cluster) within about 30 kb of the genome. The 5' flanking sequences of the five actin genes resemble each other. When constructs in which 184 bp of the 5' flanking region of each of these genes fused with lacZ were introduced into fertilized eggs, the reporter gene was expressed in muscle cells of the tailbud embryo, suggesting that the 5' flanking region of each cluster gene has promoter activity. In addition, a pair of muscle actin genes, HrMA1a and HrMA1b (HrMA1 pair), was isolated from a genomic region different from that of the HrMA2/4 cluster. The HrMA1a and HrMA1b are linked in a head-to-head arrangement on opposite strands and share a 340-bp 5' flanking sequence containing two symmetrically located TATA boxes. HrMA1a showed basically the same expression pattern as that of HrMA4a. When constructs in which the shared upstream region of HrMA1 pair fused with lacZ in either direction were microinjected into eggs, the reporter gene was expressed in muscle cells of the larval tail, suggesting a bidirectional promoter that regulates muscle-specific transcription of the HrMA1 pair. The tandem cluster of HrMA2/4 genes and the bidirectional promoter of the HrMA1 pair could expedite utilization of muscle-specific trans-acting factors. The organization of genes in the genome may play an important role in the synthesis of a large amount of actins during the process of rapid differentiation.