ACCELERATED EVOLUTION OF TRIMERESURUS-FLAVOVIRIDIS VENOM GLAND PHOSPHOLIPASE-A2 ISOZYMES

Six Trimeresurus flavoviridis (Habu snake) venom gland phospholipase A 2 (PLA2) isozyme genes were found to consist of four exons and three i ntrons and to encode proteins of 138 amino acid residues, including th e signal sequence of 16 amino acid residues. Comparison of the nucleot ide sequences showed that the introns are much more homologous than th e protein-coding regions of exons except for the signal peptide-coding region of the first exon. The numbers of nucleotide substitutions per site (K(N)) for introns are approximately one-fourth of the numbers o f nucleotide substitutions per synonymous site (K(S)) for the protein- coding regions, indicating that the introns are unusually conserved. T he absence of an apparent functional role for the introns suggests tha t the protein-coding regions, except for the signal peptide-coding dom ains, have evolved at greater substitution rates than introns. The fac t that the numbers of nucleotide substitutions per nonsynonymous site (K(A)) are close to or larger than K(S) values for relevant pairs of g enes revealed that Darwinian-type accelerated substitutions have occur red in the protein-coding regions or exons. This is compatible with th e presence of PLA2 species with diverse physiological activities in th e venom.