Structure of a human smooth muscle actin gene (aortic type) with a unique intron site

A recombinant phage containing an actin gene (lambda Ha201) was isolated from a human DNA library and the structure of the actin gene was determined. The amino acid sequences deduced from the nucleotide sequences of lambda Ha201 were compared with those of six actin isoforms; they matched those of bovine aortic smooth muscle actin, except for codon 309, which was valine (GTC) in lambda Ha201 and alanine (GCN) in bovine aortic smooth muscle actin. Southern blot hybridization experiments showed that the gene of normal human cells did not have the TaqI-sensitive site around position 309, whereas half of the genes of HUT14 cells did. These results indicate that one allele of the aortic smooth muscle actin gene in HUT14 cells has a transition point mutation (C----T) at codon 309 and that the amino acid sequences of normal human aorta and bovine smooth muscle actins are probably identical. In addition to the five introns interrupting exons at codons 150, 204, and 267, and between codons 41 and 42 and 327 and 328, which are common to skeletal muscle and cardiac muscle actin genes, the smooth muscle actin gene has two more intron sites between codons 84 and 85 and 121 and 122. The previously unreported intron site between codons 84 and 85 is unique to the smooth muscle actin gene. The intron site between codons 121 and 122 is common to beta-actin genes but is not found in other muscle actin genes. A hypothesis is proposed for the evolutionary pathway of the actin gene family.