Biotechnology Consultation Memorandum of Conference BNF No. 000007
October 4, 1994
DNA Plant Technology Corp. (DNAPT):
Jeanette Glover Glew
Subject: Delayed-ripening tomato
This meeting was intended to bring to closure DNAPT's consultation started in March 1993 (see SBJ 1319). DNAPT had previously submitted (September 16, 1994) a draft summary of the safety assessment of their delayed-ripening tomato line 1345-4.
Introduced Genetic Material
DNAPT described the identity and function of the genetic material introduced into the tomato using the Agrobacterium transformation system (summarized in the summary document). DNAPT presented DNA mapping and segregation studies which allowed them to conclude that they have 1) properly identified the sequences that were inserted into the plant genome, 2 ) ascertained that no vector or non-T-DNA sequences were found in the plant genome, 3) determined that three copies of the T-DNA were inserted at a single insertion site, and 4) determined that the insert remained stably integrated through successive generations.
Identity and Function of Expression Products Encoded by the Inserted Genetic Material
The only new protein that is expressed in the transgenic tomato is the enzyme aminoglycoside 3'-phosphotransferase II (APH(3')II), which is encoded by the kanr (nptII) gene originally isolated from transposon Tn5 isolated from E. coli. The kanr gene is used as a selectable marker. DNAPT stated that they carried out an open reading frame (ORF) analysis of the entire inserted DNA and that, while the analysis showed the presence of some potential ORFs other than the kanr and the truncated ACC synthase ORFs, they could detect no transcripts corresponding to the ORFs in the plants using appropriate double-stranded probes.
The insert also contains a fragment of the aminocyclopropane carboxylic acid (ACC) synthase gene in the sense orientation. According to DNAPT, the ACC synthase gene fragment is transcribed but not translated into a protein product; rather, transcription of the truncated gene causes suppression of the endogenous ACC synthase (Transwitch™ gene suppression technology). ACC synthase is the enzyme which catalyzes the rate-limiting conversion of S-adenosylmethionine (SAM) to ACC, the immediate precursor to ethylene. According to DNAPT, suppressing ethylene synthesis in the transgenic tomatoes causes a delayed-ripening phenotype; these tomatoes ripen normally when an external source of ethylene is applied.
Safety of the Introduced Protein
The new varieties contain only one added protein, namely APH(3')II. DNAPT noted that the safety of this protein in the development of new varieties of tomatoes has been addressed previously (21 CFR 173.170 and 21 CFR 573.130). Nonetheless, DNAPT presented data to show that the protein was expressed at very low levels (ranging from 0.72 to 2.36 ng per gram fresh weight of tomato).
DNAPT stated that tomatine levels were measured in their transgenic line 1345-4, parental line 91103-114, and a commercial fresh market variety, Sun J, grown in Southern California. According to DNAPT, tomatine levels in the transgenic tomato are unchanged from the parental variety or the traditionally-bred tomato variety (see summary document).
Concentration and Bioavailability of Important Nutrients
DNAPT noted that tomatoes make important contributions to dietary intake of vitamins A and C but are not significant sources of other dietary nutrients. DNAPT has therefore measured vitamins A and C in their transgenic and parental lines, hybrids derived from the parental line, and a non-parental commercial variety, obtained from three growing regions of the U.S. DNAPT stated that vitamin A and vitamin C levels in ripe fruits of all analyzed lines are within the ranges typical for tomatoes (see summary document).
DNAPT has concluded, in essence, that the delayed-ripening tomato variety they have developed is not significantly altered within the meaning of 21 CFR 170.30(0(2) when compared to tomato varieties with a history of safe use. At this time, based on DNAPT's description of its data and analysis, the agency, considers DNAPT's consultation on this product to be complete.
Nega Beru, Ph.D.