Contents - Faperta

Genetic Engineering of Entomopathogenic Microbes for Pest Management 261
HzNPV is better for designing recombinant clones as an insecticide targeted at the
multispecies Heliothine complex. Pesticidal properties of HaSNPV have also been
improved by deleting the ecdysteroid UDP-glucosyltransferase (egt) gene from its genome
(recombinant HaSNPV-EGTD) and incorporating an insect-selective toxin gene from the
scorpion, A. australis (recombinant HaSNPV-AaIT) (Sun et al., 2004). No differences were
observed in inactivation rates of the two recombinant HaSNPVs and their parent wild
type, HaSNPV-WT. The average half-life of HaSNPV was 0.39 to 0.90 days, and inactivation
rates correlated well with solar radiation.
The effects of expressing three PsynXIV-promoted toxin genes, mag4, sat2, and ssh1, that
encode secretable and potent neurotoxins: mu-Aga-IV from Agelenopsis aperta (Gertsch),
AsII from Anemonia sulcata Pennant, and ShI from Stichadactyla, respectively, on the ability
of the A. californica nuclear polyhedrosis virus (AcMNPV) to effectively kill its host have
been studied (Prikhod ko et al., 1998). The toxin genes mu-Aga-IV and AsII act at distinct
sites on voltage-sensitive sodium channels of insects and synergistically promote channel
opening. These toxins also had a synergistic effect against Lucilia sericata Meig. and
Spodoptera frugiperda ( J.E. Smith). Viruses expressing toxin genes under the control of the
Phsp70 promoter were more effective as bioinsecticides than under the control of the
PsynXIV promoter. The S. frugiperda and T. ni larvae were most affected by mu-Aga-IV and
AsII, respectively. When mag4 and sat2 were simultaneously expressed under the control of
Phsp70, the properties were similar to two viruses expressing each of the toxin genes individually,
except that larval feeding time was reduced by 10%, indicating a small advantage
to coproducing synergistic toxins. There is considerable scope to produce baculoviruses
expressing insect depressant toxins. Depressant insect toxin LqhIT2 from Israeli yellow
scorpion, Leiurus quinquestriatus (Ehren.) Hebreaus and BjIT2 from the black scorpion,
Buthotus judaicus Simon are nontoxic to mammals, but toxic to insects (Lester et al., 1982).
The depressant toxins are quite different from the exitant toxins, and 50 mg of LqhIT
induces complete paralysis in 50 minutes. AcDNA fragment uncoding LqhIT2 has been
synthesized (Kopeyan et al., 1990; Zlotkin et al., 1991). Full-length DNA encoding LqhIT2
has been sequenced, and contains a signal sequence of 63bp preceding the 183 base pair
coding sequence (Zilberberg, Zlotkin, and Gurevitz, 1992).
The spider toxins block neuromuscular transmission mediated by L-glutamate receptors
and the ion channels. Polypeptide neurotoxins from Hololena curta (McCook) have 36 to
38 amino acids, and result in paralysis and presynaptic blockade (Stapleton et al., 1990).
The straw mite, Pyemotes tritici (Lagreze-Fossat and Montane), produces a 27 kDa protein,
which is toxic to a number of insect species. The mite toxin TxP-1 is as effective as AaIT in
the insect nervous system (Tomalski et al., 1989). A recombinant baculovirus expressing
insect specifi c neurotoxic gene from P. tritici, lox34, has been constructed, and found to be
effective on insects (Tomalski and Miller, 1991). The baculoviruses expressing the mite
toxin gene may be more readily accepted by the public under fi eld conditions. There is
a need to understand the structure-function relationships to design toxin genes with
improved virulence, improved quantity of toxin protein, and stability during and after
production. Venom proteins from the ant, Paraponera clavata (Fab.), have also been purifi ed
(Piek et al., 1991). Toxins secreted by the wasps are highly toxic to lepidopteran larvae
(Piek, May, Spanger, 1980; Piek et al., 1989). Venoms from Colpa interrupta (F.) and Ampulex
compressa (Fab.) show activity on mammals, as well as on the cockroach, Periplaneta americana
(L.). Peptides of 9 to 18 amino acid residues of wasps and ant venoms are toxic, and
cause irreversible blocking of the insects’ central nervous system (Piek, 1990, 1991). These
can also be considered as potential genes for genetic transformation of baculoviruses.