tance to the antibiotic kanamycin and tolerance to glufosinate, was the first Bt corn to be deregulated (USDA 1999). The petition for Bt corn Transformation Event CBH-351 (StarLink) was approved in 1998 (USDA 1998). These two events expressed different kinds of plant-produced toxic proteins. The kind of toxin expressed by Event 176 has been commercially available in formulations of bacterial insecticides, but Event CBH-351 expresses a Cry toxin that has not been available commercially in insecticides. Below are highly abstracted overviews and comparisons of the assessments by USDA-APHIS for its findings of “no significant impact.” To the extent possible without access to the original research designs and test data submitted by the petitioning companies, this summary illustrates how APHIS evaluated a number of different issues associated with the biosafety of these transgenic products.
Disease Resulting Directly from the Transgenes, Their Products or Added Regulatory Sequences. For Event 176, two promoter sequences were used to allow high levels of Cry1Ab protein expression in both green tissue and pollen, which in combination were expected to be most effective in controlling ECB. Some of the DNA sequences used in transforming these plants were derived from cauliflower mosaic virus (CaMV), but the disease-causing genes of this virus were not involved. The marker gene bar from the soil bacterium Streptomyces hygroscopicus, which encodes for an enzyme resulting in glufosinate tolerance, was used as a selective marker. Plasmid pUC19 also harbors ampicillin resistance but is expressed only in a bacterium, not in plant cells.
Event CBH-351 was developed with two pUC19-based plasmids, which contained the modified cry gene and the bar gene, respectively. As in Event 176, the bar gene was cloned from S. hygroscopicus, and its expression results in whole-plant glufosinate tolerance. Event CBH-351 expresses modified Cry9c proteins, using sequences from Bacillus thuringiensis subsp. tolworthi (isolated from grain dust in the Philippines). Event CBH-351 uses regulatory regions from petunia and two plant pathogens, CaMV and Agrobacterium tumefaciens. In both cases, APHIS concluded that although pathogenic organisms were used in their development, the transgenes, their products, or added regulatory sequences do not result in plant pathogenic properties (USDA 1995, 1998). In the CBH-351 determination document, a discussion follows this conclusion, providing some information on the genetic constructs, including the number of insertion sites, the copy number and expression level of the transgenes, and evidence of stable inheritance of the transgenes in crosses of CBH-351 into several different genetic backgrounds. Unpredicted, unexplained effects