Abstract
DNA sequences have been determined for two actin genes which are closely linked in the genome of the sea urchin Strongylocentrotus purpuratus. The two genes have the same 5'-3' orientation; they were apparently formed originally by tandem gene duplication. The amino acids encoded by the two genes closely resemble those of cytoplasmic actins of mammals and slime molds and differ somewhat from those of mammalian muscle actin. Actin gene 1 had been tentatively identified earlier as the gene for an embryonic cytoplasmic actin by the homology of the 3' noncoding region with that of the cDNA of an embryonic actin mRNA from S. purpuratus. The DNA sequence of gene 1 shows presumptive signals for the initiation and termination of transcription which would govern the formation of a mature mRNA of 1.9 kilobases. Both actin genes 1 and 2 have introns in their coding regions at codons 121/122 and 204. These positions for actin introns have been reported so far only in the rat, not in lower organisms. The divergence of the sequences of these coding-region introns in the two actin genes is 66%, suggesting that the genes diverged about 90 million years ago. By contrast to the introns, the coding regions have been highly conserved; the amino acids of the two genes differ by only 1.3%, and the silent sites of the codons differ by only 12%.