Organization and phylogenetic analysis of kappa-bungarotoxin genes from Bungarus multicinctus (Taiwan banded krait).

Two genomic DNAs were isolated from the liver of Bungarus multicinctus (Taiwan banded krait) encoded kappa1-bungarotoxin and kappa3-bungarotoxin precursors, respectively. They shared virtually identical overall organization with three exons separated by two introns and a high degree of nucleotide-sequence identity with alpha-neurotoxin genes, including similar intron insertions. This suggests that kappa-neurotoxins and alpha-neurotoxins might have originated from a common ancestor. The consensus transcriptional factor binding sites within the promoter regions of these genes indicate that their transcriptions are, at least in part, regulated under the same mechanism. Comparative analyses on kappa-bungarotoxin and alpha-neurotoxin genes revealed that the protein-coding regions of exons were much more diversified than introns except for the signal peptide domain. Phylogenetic analyses on the exon and intron regions of kappa-bungarotoxin and alpha-neurotoxin genes showed that the evolution of exon regions were not in consensus with that of intron regions. The ratio of nonsynonymous to synonymous substitution is higher than 1, reflecting the occurrence of an adaptive selection during the evolution of kappa-bungarotoxins. In contrast to a conserved size of the second intron, segmental insertions and/or deletions within the first intron accelerate the evolutionary divergence of kappa- and alpha-neurotoxin genes.