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Structural
analysis of transgene rearrangements and effects on expression in
transgenic maize plants generated by particle bombardment
L.
Mehlo
We
co-transformed maize embryogenic callus with two plasmids, one carrying
the linked markers bar and gusA,
and the other carrying the Bacillus
thuringiensis cry1Ab gene. Molecular analysis of regenerated
transgenic plants revealed a diverse range of transgene structures,
including intact, truncated and internally-rearranged copies, and copies
interspersed with genomic DNA. We found rearrangements associated with
both functional and nonfunctional transgenes, and rearrangements
affecting the selected marker gene bar as well as the nonselected transgenes. Rearrangements often
caused loss of transgene expression, but some rearranged transgenes were
expressed normally. Conversely, while intact transgenes were often
expressed, we also detected some non-expressed copies, probably
reflecting epigenetic silencing mechanisms. All the transgenic lines we
studied contained multiple transgenes, generally comprising a mixed
array of intact and rearranged copies. Rearrangement is rarely given
serious consideration as a factor contributing to transgene silencing
and variable transgene expression, but our data show that undetected
rearrangements may have a major effect on transgene expression. This has
significant implications, especially in studies attempting to correlate
transgene copy number and expression levels and long-term gene
expression.
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