b-Bungarotoxin, the main presynaptic neurotoxin purified from the venom of Bungarus multicinctus, consists of two dissimilar polypeptide chains, the A chain and the B chain, cross-linked by an interchain disulfide bond. In this study, A and B chain genes isolated from the liver of B. multicinctus encoded the A and B chain precursors, respectively. Analyses of the coding regions of the A and B chain genes revealed that both consist of three exons and two introns. The sequences of all exon/intron junctions agree with the GT/AG rule. However, sequence alignment and phylogenetic analysis did not support that the evolution of A and B chain genes are closely related. Comparative analysis of A chain genes with Viperinae and Crotalinae phospholipase A 2 genes indicated that genetic divergence of the A chain and phospholipase A 2 s was in accordance with their family. Moreover, evolutionary divergence of the intron and exon regions of the A chain, as observed for phospholipase A 2 genes, was not consistent. Noticeably, the transcription of A and B chain genes may be regulated under different transcription factors as revealed by analyses of their promoter sequences. In terms of the finding that A and B chains are encoded separately by different genes, this strongly supports the view that the intact b-bungarotoxin molecules should be derived from the pairing of A and B chains after their mRNAs are translated.Keywords: A chain; B chain; b-bungarotoxin; evolutionary divergence; genetic organization.b-Bungarotoxin (b-Bgt), the main presynaptic neurotoxin purified from the venom of Bungarus multicinctus (Taiwan banded krait) [1±3], is one of the most investigated presynaptic neurotoxins from snake venoms. The toxin consists of two dissimilar polypeptide chains, the A chain with 120 amino-acid residues and the B chain with 61 residues, cross-linked by an interchain disulfide bond, and possesses a weak phospholipase A 2 (PLA 2 ) activity [4,5]. The amino-acid sequences of A chains are structurally homologous with those of PLA 2 s from snake venoms and mammalian pancreas [2,3]. B chains share sequence similarity with the trypsin inhibitor, toxin I, toxin K and dendrotoxin [6±8]. Kondo et al. [3] and Kini & Iwanaga [9] suggested that the A chain is an active subunit that is responsible for the PLA 2 enzymatic activity and neurotoxic effect of b-Bgt, while the B chain may serve as a recognition subunit of the toxin to a specific target cell membrane. Nevertheless, Benishin [10] suggested that the B chain might be responsible for the blockage of certain voltage-gated potassium channels in a manner that did not involve the A chain. Recent studies on the recombinant B chain supported this suggestion [11].The molecular structures of PLA 2 s with presynaptic neurotoxicity are quite varied. Some of these structures are single polypeptide chains, such as notexin and ammodytoxin, whereas others are binary (b-Bgt, crotoxin), ternary (taipoxin) or pentanary (textilotoxin) complexes [12]. Besides b-Bgt and the two textilotoxin D subunits, ...