Molecular cloning and evolution of the genes encoding the precursors of Taiwan cobra cardiotoxin and cardiotoxin-like basic protein

Genomic DNAs encoding the precursors of eight cardiotoxins and two cardiotoxin-like basic proteins (CLBP) were isolated from the liver of Naja naja atra (Taiwan cobra). The cardiotoxin and CLBP genes have three exons like alpha-neurotoxin precursors. The promoter regions of these genes are highly conserved and contain the consensus transcriptional factor-binding sites for TBP, NF-1, CACCC-binding site, Spl and EFII, suggesting that these genes are regulated using similar transcriptional mechanisms. The introns and flanking regions of these genes share a high degree of nucleotide sequence identity, but except for the signal peptide domain the protein-coding regions are much more diversified than introns. The ratio of nonsynonymous to synonymous substitution is higher than one, reflecting that adaptive selection occurred during the evolution of cardiotoxin and CLBP proteins. Phylogenetic trees separate CLBPs and cardiotoxins into two clusters, suggesting that the CLBP gene and the cardiotoxin gene diverged earlier before the appearance of numerous cardiotoxins and CLBP.