Venom gland transcriptomes of two elapid snakes (Bungarus multicinctus and Naja atra) and evolution of toxin genes

Abstract

BACKGROUND: Kraits (genus Bungarus) and cobras (genus Naja) are two representative toxic genera of elapids in the old world. Although they are closely related genera and both of their venoms are very toxic, the compositions of their venoms are very different. To unveil their detailed venoms and their evolutionary patterns, we constructed venom gland cDNA libraries and genomic bacterial artificial chromosome (BAC) libraries for Bungarus multicinctus and Naja atra, respectively. We sequenced about 1500 cDNA clones for each of the venom cDNA libraries and screened BAC libraries of the two snakes by blot analysis using four kinds of toxin probes; i.e., three-finger toxin (3FTx), phospholipase A2 (PLA2), kunitz-type protease inhibitor (Kunitz), and natriuretic peptide (NP). RESULTS: In total, 1092 valid expressed sequences tags (ESTs) for B. multicinctus and 1166 ESTs for N. atra were generated. About 70% of these ESTs can be annotated as snake toxin transcripts. 3FTx (64.5%) and β bungarotoxin (25.1%) comprise the main toxin classes in B. multicinctus, while 3FTx (95.8%) is the dominant toxin in N. atra. We also observed several less abundant venom families in B. multicinctus and N. atra, such as PLA2, C-type lectins, and Kunitz. Peculiarly a cluster of NP precursors with tandem NPs was detected in B. multicinctus. A total of 71 positive toxin BAC clones in B. multicinctus and N. atra were identified using four kinds of toxin probes (3FTx, PLA2, Kunitz, and NP), among which 39 3FTx-positive BACs were sequenced to reveal gene structures of 3FTx toxin genes. CONCLUSIONS: Based on the toxin ESTs and 3FTx gene sequences, the major components of B. multicinctus venom transcriptome are neurotoxins, including long chain alpha neurotoxins (α-ntx) and the recently originated β bungarotoxin, whereas the N. atra venom transcriptome mainly contains 3FTxs with cytotoxicity and neurotoxicity (short chain α-ntx). The data also revealed that tandem duplications contributed the most to the expansion of toxin multigene families. Analysis of nonsynonymous to synonymous nucleotide substitution rate ratios (dN/dS) indicates that not only multigene toxin families but also other less abundant toxins might have been under rapid diversifying evolution.