In an airport first-class lounge, VIP may stand for very importantpassengers, but to molecular biologist Juan Estruch the letters Vipsignify vegetative insecticidal protein.
Estruch is a senior scientist in the Insect Control Group of CibaAgricultural Biotechnology, in Research Triangle Park, N.C. He isresponsible for identifying, cloning and screening the rapidlyexpanding roster of Vips secreted by the well-known bug-killing soilbacterium, Bacillus thuringiensis (Bt).
Summer sufferers from mosquitoes know and love B. thuringiensisisraelensis as the ecology-friendly spray that cuts down on theinsect's seasonal infestations. The bacterium makes crystals of Btendotoxin during the late-stage sporulation phase of its life cycle.
Ingested by unsuspecting insects, these endotoxins kill by punchingholes in their midgut. Bts of varying molecular configurations arewidely used to control insect pests in agriculture as well as onmosquito breeding sites.
But these endotoxins have their limitations: They scarcely bother, letalone kill many lepidopterans _ that is, the caterpillars from whichcertain butterflies and moths spring. Which is where the Vip genesand their protein products come in.
Estruch is first author of a paper in the latest Proceedings of theNational Academy of Sciences, (PNAS) dated May 28, 1996, titled"Vip3A, a novel Bacillus thuringiensis vegetative insecticidal proteinwith a wide spectrum of activities against lepidopteran insects."
What makes Vips different is that word "vegetative." Estruchexplained to BioWorld Today: "Like human beings, bacillus, and anybacteria, have several stages during their growth. When young, theygrow rapidly, and are very healthy. As adults, they stop growing, andusually enter the sporulation of old age."
Soluble Bt Insect Killers Join Crystalline
It's during this spore-making stage, late in bacterial development,that Bt produces its familiar crystalline insecticidal endotoxin.Soluble Vips, on the other hand, arise from its youthful, vegetativestage of life.
"This particular protein," Estruch said, "which contains no sequencehomologies with known proteins, is very active against the fallarmyworm (Spodoptera frugiperda) and black cutworm (Agrotisipsilon), so these are the two main insects we work on."
As well they might.
Armyworms travel overland to new feeding grounds like vast infantryformations. Cutworms attack more than 50 crops, by night, includingmaize, tomatoes and beans; it severs their roots and shoots nearground level. "This pest is difficult to control," the PNAS paperpoints out, "because by the time the infestations are apparent, thesusceptible stages [i.e., larvae] are past, and damage may already beserious and irreversible."
"Our ultimate goal," Estruch said, "is always to introduce those Vipgenes into plants. Our main interest is corn." He and his co-authorsare working to create corn transgenic for the Vip gene and its borer-and cutworm-specific protein. "The market potential for this oneproduct," he surmises, "must be over a billion dollars, for sure."
Estruch predicted: "The second-generation transgenic plants that willcontain these Vip genes will probably come to market in a fewyears," and added, "We have continued to discover many otherinsecticidal proteins in the same Bacillus thuringiensis."
Meanwhile, this year, Ciba's seed unit launched its first-generationtransgenic corn seed, "Maximizer on the U.S. market." This hybrid isarmed with "KnockOut" _ Bt endotoxin genes targeting theEuropean corn borer (Ostrinia nubilalis), against which Vips areimpotent.
In 1994 field trials, Ciba now is telling prospective customers:
* average yield improvement over conventional hybrids was 42.7bushels per acre;
* corn borer infestation in 1994 caused Maximizer to be 0.75 percentdrier than other hybrids;
* it had two fewer corn borer entry holes per stalk;
* every place the borer chews into the plant provides a place for otherinsects to move in.
Why should a soil bacterium such as B. thuringiensis deal out such abroad spectrum of insect-slaying proteins?
"There are a lot of opinions," Estruch said, "but I don't thinkanybody has the right answer. Some think it gives the bacillus anecological niche, developing proteins to kill insects so it can liveupon the insects."
This speculation, he observed, "conflicts with the law of parasites,that a parasite tries not to kill the organism on which it lives. Butthere is no good alternative explanation." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.