Transgenic plants have now become a commercial reality and genetic transformation is routinely done in most crop species. However, achieving a high expression of the introduced foreign gene in plant cells is still a challenging task. Especially in those projects aiming to develop plants as "biofactories" to produce novel enzymes, pharmaceuticals and industrial compounds, it is critical that introduced genes churn out the corresponding proteins in large amounts to make the venture economically feasible.

The level of gene expression is, in part, a function of the promoter to which the coding region of the gene is fused. Use of the most popular promoter in plant molecular biology research, the 35S promoter from cauliflower mosaic virus (CaMV), usually results in production of the foreign gene product at rates of less than one percent of the total protein. Improvements to the CaMV 35S promoter such as the duplication of certain sequences and the addition of enhancers have boosted expression, but for some applications, the levels need to be still higher.

Now, a novel promoter developed at the laboratory of Stan Gelvin at Purdue University may come to the aid of plant scientists frustrated by low expression levels of introduced genes. In a "head-to-head" test, the new super-promoter resulted in 156-fold more GUS activity than did the -800 CaMV 35S promoter of pBI121. Although actual protein levels were not measured and other versions of the 35S promoter are somewhat stronger than the one used in the test, the results are nonetheless quite impressive.

This promoter is derived from the soil-borne plant pathogen Agrobacterium tumefaciens which causes crown gall disease and serves as a common vector to introduce genes into plant cells. Gelvin and associates experimented with various combinations of regulatory sequences from two genes from this bacteria: octopine synthase (ocs) and mannopine synthase (mas). Among the many combinations they tested, a hybrid promoter combining a triple repeat of the ocs activator sequence along with mas activator elements fused to the mas promoter did the trick! Termed "(Aocs)3AmasPmas", the promoter is also referred to as construction #5 in the publication where it is described in detail (1).

In transformed tobacco cells, expression of the GUS marker gene driven by this promoter allowed detection of ten-fold more transformants than when the CaMV 35S promoter was used. It is not likely that the new promoter affects transformation efficiency, but the elevated level of marker expression made it easier to identify a higher proportion of transformed cells. Expression levels were high in leaf, stem and root and in most cell types. The promoter was also very active in cassava and cowpea, two crop plants which have appeared difficult to transform using Agrobacterium.

In a paper soon to be published in the May issue of Plant Cell, Gelvin's group describes their success in the use of this promoter to study very early transcription of introduced genes (within 18 hours) in tobacco and maize. The CaMV 35S promoter was too weak to show any activity at this time. Other researchers at various laboratories are now testing the new promoter to see if it would help boost levels of expression of their introduced genes in plants.

The "Super-Promoter" has been licensed by Purdue University to Biotechnology Research and Development Corporation (BRDC) of Peoria, Illinois. Those interested in obtaining the promoter can contact Grant Brewen at BRDC (tel: 1-309-688-1188; fax: 1-309-688-1292). Technical questions should be addressed to Stan Gelvin at 317-494-4939; gelvin@bilbo.bio.purdue.edu.

Reference
(1) Ni, M., Cui, D., Einstein, J., Narasimhulu, S., Vergara, C.E., and Gelvin, S.B. (1995). Plant J. 7, 661-676.

C. S. Prakash
Center for Plant Biotechnology Research
Tuskegee University
prakash@acd.tusk.edu