Tobacco ribosomal DNA spacer element stimulates amplification and expression of heterologous genes

Here we show that the cis-acting genetic element aps (amplification-promoti ng sequence), isolated from the nontranscribed spacer region of tobacco rib osomal DNA (rDNA), increases the level of expression of recombinant protein s. Transgenic tobacco plants, transformed with expression cassettes contain ing the herbicide-resistant acetolactate synthase (hr-ALS) gene or the gree n fluorescent protein (GFP) gene fused to the aps sequence, had greater lev els of corresponding messenger RNAs (mRNAs) and proteins compared to transf ormants lacking aps. Analysis of transgenic plants showed that aps increase d the copy number and transcription of the adjacent heterologous genes and, in the case of hr-ALS, enhanced the herbicide resistance phenotype. Both t he increased transgene copy number and enhanced expression were stably inhe rited. These data provide the first evidence that the aps sequence can be u sed for gene amplification in transgenic plants and possibly other multicel lular organisms.