The nucleotide sequences of four genes encoding Trimeresurus gramineus (green habu snake, crotalinae) venom gland phospholipase A(2) (PLA2; phosphatidylcholine 2-acylhydrolase, EC 3.1.1.4) isozymes were compared internally and externally with those of sis genes encoding Trimeresurus flavoviridis (habu snake, crotalinae) venom gland PLA2 isozymes, The numbers of nucleotide substitutions per site (K-N) for the noncoding regions including introns were one-third to one-eighth of the numbers of nucleotide substitutions per synonymous site (K-S) for the protein-coding regions of exons, indicating that the noncoding regions are much more conserved than the protein-coding regions, The K-N values for the introns were found to be nearly equivalent to those of introns of T. gramineus and T. flavoviridis TATA box-binding protein genes, which are assumed to be a general (nonvenomous) gene, Thus, it is evident that the introns of venom gland PLA2 isozyme genes have evolved at a similar rate to those of nonvenomous genes, The numbers of nucleotide substitutions per nonsynonymous site (K-A) were close to or larger than the K-S values for the protein-coding regions in venom gland PLA2 isozyme genes, All of the data combined reveal that Darwinian-type accelerated evolution has universally occurred only in the protein-coding regions of crotalinae snake venom PLA2 isozyme genes.
