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We present an in-depth analysis of the structural and functional properties of Imperatoxin I (IpTxi), an approximately 15-kDa protein from the venom of the scorpion Pandinus imperator that inhibits Ca2+ release channel/ryanodine receptor (RyR) activity (Valdivia, H. H., Kirby, M. S., Lederer, W. J., and Coronado, R. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 12185-12189). A cDNA library was prepared from the venomous glands of this scorpion and used to clone the gene encoding IpTxi. From a single continuous messenger RNA, the information coding for the toxin is translated into two mature polypeptide subunits after elimination of a basic pentapeptide. The IpTxi dimer consists of a large subunit (104-amino acid residues) with phospholipase A2 (PLA2) activity covalently linked by a disulfide bond to a smaller (27 amino acid residues), structurally unrelated subunit. Thus, IpTxi is a heterodimeric protein with lipolytic action, a property that is only shared with beta-bungarotoxins, a group of neurotoxins from snake venoms. The enzymatic subunit of IpTxi is highly homologous to PLA2 from bee (Apis mellifera) and lizard (Heloderma horridum) venoms. The small subunit has no significant similarity to any other known peptide, including members of the Kunitz protease inhibitors superfamily that target the lipolytic effect of beta-bungarotoxins. A synthetic peptide with amino acid sequence identical to that of the small subunit failed to inhibit RyR. On the other hand, treatment of IpTxi with p-bromophenacylbromide, a specific inhibitor of PLA2 activity, greatly reduced the capacity of IpTxi to inhibit RyRs. These results suggested that a lipid product of PLA2 activity, more than a direct IpTxi-RyR interaction, was responsible for RyR inhibition.

A peptide (toxin II‐10), shown to be a Na+channel blocker, was purified from the venom of the scorpion Centruroidesnoxius Hoffmann and sequenced by Edman degradation. It has 66 amino acid residues with the C‐terminal residue (asparagine) amidated, as demonstrated by mass spectrometry. In addition, we report the cloning and the nucleotide sequence of the cDNA (CngtV) that codes for this toxin. We discuss the mechanism for processing the precursor peptide to its final form and compare the primary structure to that of other Na−channel toxins. Two distinct groups of toxins seem to emerge from this comparison, suggesting a structure—function relationship of these peptides towards the recognition of either mammalian or insect tissues.

Rhizobium phaseoli lacks glutamate dehydrogenase (GDH) and assimilates ammonium by the glutamine synthetase-glutamate synthase pathway. A strain of R. phaseoli harboring the Escherichia coli GDH structural gene (gdhA) was constructed. GDH activity was expressed in R. phaseoli in the free-living state and in symbiosis. Nodules with bacteroids that expressed GDH activity had severe impairment of nitrogen fixation. Also, R. phaseoli cells that lost GDH activity and assimilated ammonium by the glutamine synthetase-glutamate synthase pathway preferentially nodulated Phaseolus vulgaris.

The gene encoding toxin gamma from the scorpion, Tityus serrulatus, was amplified by PCR from genomic DNA employing synthetic oligonucleotides designed from the reported cDNA sequence. The nucleotide sequence of this gene reveals the presence of an intron of 475 base pairs (bp) which interrupts the region that encodes the signal peptide of the precursor toxin. A comparison of the intron boundary sequences of the gamma toxin gene with ones from other arachnid genes is also presented.