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

Here we show that the cis-acting genetic element aps (amplification-promoting sequence), isolated from the nontranscribed spacer region of tobacco ribosomal DNA (rDNA), increases the level of expression of recombinant proteins. Transgenic tobacco plants, transformed with expression cassettes containing the herbicide-resistant acetolactate synthase (hr-ALS) gene or the green fluorescent protein (GFP) gene fused to the aps sequence, had greater levels of corresponding messenger RNAs (mRNAs) and proteins compared to transformants lacking aps. Analysis of transgenic plants showed that aps increased the copy number and transcription of the adjacent heterologous genes and, in the case of hr-ALS, enhanced the herbicide resistance phenotype. Both the increased transgene copy number and enhanced expression were stably inherited. These data provide the first evidence that the aps sequence can be used for gene amplification in transgenic plants and possibly other multicellular organisms.