Cloning and characterization of fish promoters is important for gene transfer research and studies of cell differentiation and regulation of gene expression. Identification of the genomic structure of fish genes provides valuable sequence information

for gene transfer research and studies of cell differentiation and regulation of gene expression. Identification of the genomic structure of fish genes provides valuable sequence information and adds to understanding of their molecular evolution. We aimed at cloning rainbow trout regulatory sequences that direct expression of genes in skeletal muscle. To address this task, we chose α-actin, myosin light regulatory chain and tropomyosin. Actins are highly conserved structural proteins. Being components of contractile structures and cytoskeleton, actins are distributed ubiquitously. Of three major vertebrate actin groups (α, β and γ), α-actins are specific for striated muscle. Cardiac isoforms are predominant in heart, whereas skeletal αactins are found in both skeletal and cardiac muscle. In pufferfish (Fugu rubripes), nine actin genes have been cloned, of which six were identified as α-isoforms (two muscle, three cardiac and one anomalous testis α-actin; Venkatesh et al., 1996). α-Actin genes have been cloned from three more fish species, zebrafish (Danio rerio; Higashijima et al., 1997), medaka (Oryzias latipes; Kusakabe et al., 1999) and channel catfish (Ictalurus punctatus; Kim et al., 2000), and their promoters have been functionally assessed. The myosin complex is a hexamer of two heavy and four light chains, of which regulatory chain is required for calcium binding. This gene has been cloned from one teleost species, zebrafish (Xu et al., 2000). Both α-actin and MLC2 promoters have been an important model for identification of muscle-specific regulatory elements in vertebrates. In skeletal muscle, tropomyosin is a dimer of α and β chains, mediating interaction between the troponin complex and actin that is required for regulation of contraction. Unlike actin and myosin, tissue-specific isoforms of mammalian tropomyosin are encoded by a single gene, and multiple proteins arise due to usage of different promoters and alternative splicing. Promoters of mammalian α-tropomyosins are characterized by 601 The Journal of Experimental Biology 206, 601-608 © 2003 The Company of Biologists Ltd doi:10.1242/jeb.00116