Ammodytoxin C gene helps to elucidate the irregular structure of Crotalinae group II phospholipase A2 genes

Ammodytoxin C is a presynaptically neurotoxic phospholipase A2 (PLA2) expressed in the venom glands of Vipera ammodytes (subfamily Viperinae). The gene spans more than 4 kb and consists of five exons and four introns characteristic of group II phospholipase A2 genes. The first exon encodes the 5' untranslated region, the second exon encodes most of the signal peptide, while exons 3-5 encode three parts of the mature protein. Comparison of the Crotalinae and Viperinae PLA2 genes has shown that Crotalinae PLA2 retain the first intron in their mRNAs. The apparent cause of this retention is a deletion of 40 bp in the first exon of PLA2 genes of the subfamily Crotalinae, which prevents splicing of the first intron. Analysis of the secondary structure of the pre-mRNA of the ammodytoxin C gene has shown that the first exon is able to form an intra-exon hairpin which is absent in Crotalinae PLA2 pre-mRNAs. Our results indicate that this intra-exon hairpin structure is essential for the splicing of the retained first intron. Contrary to the predictions of the neutral theory of molecular evolution, the introns of all known snake venom PLA2 genes are conserved up to 90%, that is considerably more than the exons. Consequently it is proposed that highly conserved introns, in multigene families, which evolve under positive Darwinian selection, may have an important role in enabling homologous recombination.