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R. Latham et al and genomic DNA as well as probable additional rearrange ments that were not ivestigated further [31]. to have caused gene silencing of the entire glutelin gene fam ily [40. Obviously, the more the present scrambling is, the The sequence of a functional transgene insertion site se higher the probability that an aberrant phenotype will result sulting from particdle bombardment has therefore never been is. Alternatively, complex insertion sites may generate aber definitively compared to its undisrupted site of insertion, e rant transcripts coding for fusion proteins that can also lead ther in the scientific literature or in applications submitted to mutant phenotypes to US regalators Consequently, the minimal extent of mu- tation possible at a functional particle bombardment inser- rare. To our knowledge, only two papers describe transcrip tion site is unknown. Due to the small number of events tion patterns at transgene integration sites and both stud- analysed leven partially), ㏂y conclusions regarding parti- led events approved for commercial release [41, 42]. Al- cle bombardment insertion events can only be provisional. though neither focussed on flanking DNA, both showed ev- However, it appears that transgene integration resulting from idence for aberrant transcription. At the Soybean Roundup particle bombardment is usaally or always accompanied by Ready 40-3-2 locus, transcription originating in the trans substantial disruption of plant DNA and insertion of super-ene continues into scrambled DNA at the end of the lo- fluous DNA Studies of transcription patterns at insertion sites are cus. This is a consequence of inefficient transcription termi- Given the relative lack of research describing insertion nation by the nopaline synthase transcription terminator sites resulting from particle bombardment, it is interesting termination sequence widely used in transgenic plants 142. that there is a single report of the insertion of contaminating Rang et al predicted that the oversined transcripts detected at bacterial dromosonsal DNA adiacent to transgene 33。It the Roundup Ready locus might express fusion proteins con. is as yet too early to say whether insertion of contaminating taining the EPSPS gene and the unidentified sequence flank ing the scrambled transgene [42. However, they made no attempt to detect any protein product. In the second report, several abundant and oversized mRNA transcripts originat- ing in the transgene were detected by Northern blotting of a origin and sig- It is apparent that small and large-scake deletions, rearrangenificance of these aberrant viral transcripts was never inves DNA is a common outcome of particle bombardment MOLECULAR CONSEQUENCES OF TRANSGENE INSERTION irus-resistant papaya line. The ments of plant DNA, and insertion of superfluous DNA are tigated 41] each common occurrences at Agrobacterim-mediated trans- There are other mechanisms by which insertion-site mu gene insertion sites Particle bombardment insertion sites, tations may affiect plant phenotypes. Sequences carried on however, appear always to be associated with genome disrupT-DNAs can alter the expression of neighbouring genes at least 12 Kbp from the transgene (18, 35, 43. Such distant ef tion, rearrangements, and superfluous DNA Mutations at insertion sites have the potential to result infects are thought to be mediated by multiple promoters and inadvertent loss, acquisition, or misexpression of important therefore it is plausible that scrambled or complex transgeme traits, in part because transgenes insert into or near fune insertion sites that accumulate multiple promoter sequences tional gene sequences, In the plant species most studied may also influence the expression of linked genes thaliana and nce), approximately 27%-40% of T-DNA inser- tions disrupt known gene sequences [15, 24, 34-37).Large (bacterial origins of replication in particular), or antibiotic scale studies of insertion patterns of transgenes delivered by resistance genes accidentally inserting adjacent to the trans particle bombardment have never been conducted in any gene may significantly enhance the probabality of horizon- species. Deletions or rearrangements associated with trans tgene transfer. Awailability of sequence homology is con- gene insertion further increase the likelihood of alterations sidered one of the major obstacles to horizontal gene trans- to the plant phenotype. Among many examples, the 78 Kbp fer from plants to bacteria. By providing adjacent regions of deleticsa recorded in A สǐalana resulted in loss of 13 genes bacterial sequence homology, researchers have shown that and disruption of two others (16). The 1,980 bp deletion also horirontal gene transfer from bacterial replicons to bacterial noted earlier resulted simultaneously in upregulation and al chromosomes can be elevated 10% fold 144, 451.Most (US ap tered trancript sizes of an adjacent gene 118) Lastly, bacterial chromosomal DNA, plasmid sequences proved) commercial transgenic cultivars analysed in a previ Gene disruption and dedetion are not the only mecha ous publication had insertions of superfluous bacterial DNA nisms by which transgene insertion may affect the phenotyp athe insertion site 146 of a transgenic plant. When transgene insertion is associated s or insertion of superfluous DNA then GENOME-WIDE MUTATIONS justaposition of promoter sequences and coding fragments may lead to sense or antisense transcripts which, simalarly The second class of mutations associated with plant trans- to siRNAs and miRNAs, can interfere with the expression of formation are genome-wide mutations. These are not nec- genes containing homologous or similar sequences 138, 39 essarily genetically linked to the transgene insertion site but A naturally occurring instance of this phenomenon has been arise as a consequence of tissue culture, probably Agrobac- reported in the nontransgenic nce I wg utelin content muta- terrum anfection and possibly particle bombardment and an- tion. Here, a deletion resulted in transcription into a neigh tibitic use 13, 10, 47, 481- There are 5 studies in which e- bouring member of the glutelin gene family and was thought searchers have attempted to quantify mutations introduced
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