Trimeresurus flavoviridis venom gland phospholipase A 2 isozymes genes have evolved via accelerated substitutions

As a step towards understanding the structure and function of phospholipases A 2 (PLA 2 s), five cDNAs encoding Trimeresurus flavoviridis venom gland PLA 2 isozymes have been sequenced. They revealed that the 5′ and 3′ un translated regions are much more homologous than the protein‐coding regions and that mbase substitutions have occurred at similar rates for the three positions of codons in the protein‐coding regions. Such novel findings are of great interest from the viewpoint of molecular evolution. To gain a further insight into this evolutional phenomenon, we have isolated and sequenced six T. flavoviridis PLA 2 isozyme genes. Each gene consisted of four exons and three introns and encoded protein of 138 amino‐acid residues, including the signal sequence of 16 amino‐acid residues. The introns were much more homologous than the protein‐coding regions of exons except for the signal peptide‐coding region of the first exon. The absence of apparent functional role in the introns suggested that the protein‐coding regions, expect for the signal peptide‐coding domains, have evolved at greater substitution rates than introns. The fact that the numbers of nucleotide substitutions per non‐synonymous site are close to or larger than the number of nucleotide substitutions per synonymous site for relevant pairs of genes revealed that Darwinian‐type accelerated substitutions have occurred in the protein‐coding regions of exons. This is compatible with the presence of PLA 2 species with diverse physiological activities in the venom.