Four distinct actin genes of the sea urchin Strongylocentrotus purpuratus have been isolated from a recombinant Charon 4 phage library of genomic DNA. The four genes differ considerably from each other in many of their restriction sites. Two of the four genes are closely linked; they are present in the same fragment of cloned DNA. This fragment has been extensively mapped, and some parts of the DNA have been sequenced. The two linked genes are oriented in the same direction, separated by 7.5 kb of DNA. One has an intron following the CAG that codes for the glutamine residue at position 121 in the amino acid sequence of actin. This represents the fifth distinct site at which introns have been found in actin genes, suggesting that the primordial actin gene had at least 6 exons and 5 introns. The actin genes form a distinctive family in which most introns have apparently been precisely excised from the genes.
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