The genomic nucleotide sequences of two differentially expressed actin-coding genes from the sea star Pisaster...

The genomic sequences of two differentially expressed actin genes from the sea star Pisaster ochraceus are reported. The cytoplasmic actin gene (Cy) is expressed in eggs and early development. The muscle actin gene (M) is expressed in tube feet and testes. Both genes contain an 1125-nucleotide coding region interrupted by three introns at codons 41, 121 and 204. Gene M contains two additional introns at codons 150 and 267. The intron position at codon 150, although present in higher vertebrate actins, has not been reported in actin genes from invertebrates. The M gene coding region has 89.5% nucleotide homology to the Cy gene, and differs from the Cy actin gene in 13 of 375 amino acids (aa), 11 of which are found in the C-terminal half of the gene. The C-terminal half of the M gene contains a significant number of muscle isotype codons. Even though there is only 1 aa change in the first 150 codons, there have been limited substitutions at many four-fold degenerate sites which may indicate selection pressure upon the secondary structure of the mRNA and/or a biased codon usage. Variant CCAAT, TATA, and poly(A)-addition signals have been identified in the 5' and 3' flanking regions. The presence of 5' and 3' splice junction sequences in the 5' flanking region of the Cy gene suggests the potential for an intron there.