Abstract
Transgenes stably integrated into chromosome are often silenced, a phenomena is called gene silencing. Gene silencing has been attributed to several factors, including differences in chromosome position, repeat sequences, and copy number. We transformed Arabidopsis plants with pBI121 (CaMV35S-GUS and NOS-NPTII) using Agrobacterium-mediated transformation, and carefully selected ten lines containing single 'complete' (intact, non-truncated, non-rearranged) copies of the transgene located at different chromosome positions. These lines showed very similar levels of transgene expression, and gene silencing was not observed. In a subsequent study, intercrosses of isolated lines were performed to generate plants harboring one to eight copies of the transgene. One to five copies of the GUS gene were introduced in the genome, positive correlation between gene copy number and GUS activity was observed. However, six or more copies of the GUS gene resulted in silencing.
