Genomic organization of three novel toxins from the scorpion Buthus martensi Karsch that are active on potassium channels

The cDNA and genomic DNA of three novel toxins from the scorpion Buthus mar tensi Karsch that are active on K+ channels, designated BmKTX (where KTX is kaliotoxin), BmTX1 and BmTX2, were cloned and sequenced. On the basis of t heir known amino acid sequences, gene-specific primers for 3' and 5' rapid amplification of cDNA ends (RACE) were designed and synthesized. By overlap ping the two partial cDNA sequences obtained by 3' and 5' RACE, their full- length cDNA sequences were completed. BmKTX encodes a signal peptide of 22 amino acid residues and a mature toxin of 38 residues, whereas BmTX1 and Bm TX2 encode signal peptides of 20 and 21 residues respectively and a mature toxin of 38 residues for each. Their cDNA-deduced amino acid sequences were totally consistent with those determined except that the C-terminus of BmK TX had an additional Gly residue, which was removed during post-translation al processing and was indispensable for the amidation of its C-terminal Lys residue. In addition, the first deduced amino acid for both BmTX1 and BmTX 2 is Gin instead of pyro-Glu in the reported toxins, which obviously also u ndergoes post-translational processing. The genomic DNA species of these th ree toxins were also amplified by PCR, then cloned and sequenced. They all consisted of two exons disrupted by a small single intron. All of these int rons were inserted within the signal peptides at position -6 for BmKTX and at position -5 for both BmTX1 and BmTX2 upstream of the mature toxins, and consisted of 87, 87 and 80 bp respectively.