Molecular evolution of two actin genes from carrot.

We have isolated and sequenced two full-length cDNA clones encoding actin from carrot. The two carrot clones are almost identical at the nucleotide level, and are quite homologous to each other and to other plant actins at the amino acid level. In those regions where amino acid variation exists between the two genes from carrot, the differences have arisen from very simple changes at the nucleotide level. The most common changes are nucleotide insertion(s) coupled to the deletion of a different nucleotide(s) nearby in the DNA sequence, resulting in the restoration of the proper reading frame for the protein; thus, these changes can be viewed as multiple or coupled frameshift mutations. There are almost no base substitutions between the two carrot genes. In contrast to this, when the carrot actin nucleotide sequences are compared to those of a soybean actin gene or a maize actin gene, many base substitutions are observed (ca, 21.8% and 23.5%), more than half of which are third base changes which do not alter the protein sequence. At the amino acid level, both carrot genes show greater similarity to maize actin than they do to soybean actin, thus reinforcing the idea that plant actin genes diverged from a single common ancestral actin gene prior to the divergence of monocots and dicots.