We would like to address the suggestion of Matzke et al (1) and
others that pararetroviral promoters such as the CaMV 35S are not exotic to
plant genomes as they already contain many integrated pararetroviral sequences.
The crucial question is whether the CaMV 35S promoter in transgenic constructs
poses special risks. We would like to draw attention to several publications
that are relevant to this issue.
Kumpatla and Hall (2) analyzed a transgenic rice locus and confirmed
that fragmentation and recombination occur frequently within the CaMV 35S
promoter, but not in the wheat plant ubiquitin promoter used in another
transgenic cassette. This indicates that the CaMV promoter is not like any
other promoter. Six out of seven recombination junctions in the CaMV promoter
map near the 19 basepair palindrome identified as a recombination hotspot by
Kohli et al (3).
The conventional wisdom among plant molecular geneticists is that plant
promoters, such as the CaMV 35S, are not active in animals (4). In fact, the
CaMV 35S promoter was found to support high levels of reporter gene expression
in mature Xenopus oocytes (5), and to give very efficient transcription
in extracts of HeLa cell nuclei (6). The CaMV promoter worked at least as well
as the SV40 promoter in Xenopus oocytes, and better than the major late
promoter of the adenovirus-2 in HeLa cell extracts.
So, while the CaMV is specific for plants in the cruciferae family, its
isolated promoter is promiscuous across domains and kingdoms of living
organisms. It is the genetic (and evolutionary) context that makes all the
difference. There is no justification for claiming that the promoter in
transgenic constructs is as safe as the promoter in the intact viral genome,
nor to consider it equivalent to the promoter of proviral sequences in the
Matzke, M.A. , Mette, M.F. and Aufsatz, W. More on CaMV, Nature Biotechnology 2000: 18: 579.
Kumpatla, S.P. and Hall, T.C. (1999). Organizational complexity of a
rice transgenic locus susceptible to methylation-based silencing. IUBMB
Life 1999: 48: 459-467.
Hull, R., Covey, S.N. and Dale, P. Microbial Ecology in Health and
Disease (in press).
Kohli, A., Griffiths, S., Palacios, N., Twyman, R.M., Vain, P.,
Laurie, D.A. and Christou, P. Molecular characterization of transforming
plasmid rearrangements in transgenic rice reveals a recombination hotspot in
the CaMV 35S promoter and confirms the predominance of microhomology mediated
recombination. Plant J. 1999: 17: 591-601.
Ballas,N., Broido, S., Soreq, H., and Loyter, A. (1989). Efficient
functioning of plant promoters and poly(A) sites in Xenopus oocytes Nucl
Acids Res 1989: 17: 7891-903.
Burke, C, Yu X.B., Marchitelli, L.., Davis, E.A., Ackerman, S.
(1990). Transcription factor IIA of wheat and human function similarly with
plant and animal viral promoters. Nucleic Acids Res 1990: 18:
Angela Ryan Institute of Science in
Society and Dept. of Biological Sciences Open University Milton Keynes,
MK 7 6AA, UK
Joe Cummins Dept. of Plant
Sciences, University of Western Ontario Canada