Different 3' End Regions Strongly Influence the Level of Gene Expression in Plant Cells

We have investigated the functional role of a 3' end region on the expression of a reporter gene in plant cells. In stably transformed plants, expression of the reporter gene without a plant gene 3' end is variable and depends on the fortuitous presence of polyadenylation signals in the downstream sequences. When the reporter gene is flanked by pBR322 DNA, 3'-processing and polyadenylation occurs at (a) cryptic site(s) within these vector sequences. Using a transient gene expression system, we present a deletion analysis of the 3' end of the octopine synthase gene showing that the most proximal polyadenylation signal per se is not sufficient to ensure expression but that a downstream (G)T-rich sequence is also required. Optimal expression of the fusion gene requires more than 98 base pairs and at most 142 base pairs downstream from the most distal polyadenylation site. We analyzed the expression of chimeric genes with 3' end sequences originating from different plant genes. In the transient expression assay, all constructs direct similar neomycin phosphotransferase II activities. However, in stably transformed tissue, the gene constructs displayed characteristic expression levels which varied as much as 60-fold. This result suggests a role for 3' end sequences in post-transcriptional processes such as efficiency of 3'-processing and/or mRNA stability.