Joe Cummins - Dept. of Plant Sciences,
University of Western Ontario, Ontario, Canada
Mae-Wan Ho and Angela Ryan, Department of Biological Sciences, Open
University, Walton Hall, Milton Keynes, MK7 6AA
(To appear in Nature Biotechnology April 2000)
This is a rebuttal to an article in Nature Biotechnology (Jan.
2000) attacking an earlier article, now published (Ho, M.W., Ryan, A.,
Cummins, J. (1999) The cauliflower mosaic viral promoter a recipe
for disaster? Microbial Ecology in Health and Disease 11,
In your account (Jan. 2000) (1) of our pre-publication manuscript, you
quote the criticisms but ignore completely our full rebuttal, which was
posted on the web last November. We shall outline the main points made in
reply to the criticisms. The full details and references are available on
our website (2).
Our manuscript (3) reviews and synthesizes the scientific literature on
the 35S promoter of the cauliflower mosaic virus (CaMV) used to give
constitutive over-expression of transgenes in practically all GM crops
already commercialized or undergoing field trials. The promoter functions
efficiently in all plants, as well as green algae, yeast and E. coli.
It has a modular structure, with parts common to, and interchangeable with
promoters of other plant and animal viruses. It also has a recombination
hotspot, flanked by multiple motifs involved in recombination, similar to
other recombination hotspots including the borders of the Agrobacterium
T DNA vector most frequently used in making transgenic plants. The
suspected mechanism of recombination double-stranded DNA
break-repair - requires little or no DNA sequence homologies. Finally,
recombination between viral transgenes and infecting viruses has been
demonstrated in the laboratory (4).
The findings suggest that transgenic constructs with the CaMV 35S
promoter may be structurally unstable and prone to horizontal gene
transfer and recombination. The potential hazards are mutagenesis,
carcinogenesis, reactivation of dormant viruses and generation of new
viruses. These considerations are especially relevant in the light of
recent findings that certain transgenic potatoes - containing the CaMV 35S
promoter - may be unsafe for young rats, and that a significant part of
the effects may be due to "the construct or the genetic
transformation (or both)" (5).
Our critics believe the CaMV 35S promoter is not harmful because people
have been eating the virus in infected cabbages and cauliflower for many
years. What we have been consuming is predominantly intact virus and not
naked viral genomes. Naked viral genomes have been found to give
full-blown infections in non-host species that are not susceptible to the
intact virus (6). Moreover, the 35S promoter in the CaMV is a stable,
integral part of the virus, and cannot be compared to the 35S promoter in
artificial transgenic constructs. Artificial constructs are well-known to
be structurally unstable (7). We know that the 35S promoter in the virus
does not transfer into genomes because pararetroviruses, such as CaMV, do
not integrate into host genomes to complete their lifecycle; and viral
replication takes place in the cytoplasm (8). But that says nothing about
the 35S promoter in transgenic constructs that are integrated into host
Proviral sequences are present in all genomes, and as all viral
promoters are modular, and have at least one module the TATA box -
in common, if not more, it is not inconceivable that the 35S promoter in
transgenic constructs can reactivate dormant viruses or generate new
viruses by recombination. The CaMV 35S promoter has been joined
artificially to the cDNAs of a wide range of viral genomes, and infectious
viruses produced in the laboratory (9). There is also evidence that
proviral sequence in the genome can be reactivated (10).
The fact that plants are "loaded" with potentially mobile
elements can only make things worse. Most, if not all of the elements will
have been tamed in the course of evolution and hence no longer
mobile. But integration of transgenic constructs containing the 35S
promoter may mobilize the elements. The elements may in turn provide
helper-functions to destabilize the transgenic DNA, and may also serve as
substrates for recombination to generate more exotic invasive elements.
In signing on to the International Biosafety Protocol in Montreal in
January, more than 150 governments agreed to implement the precautionary
principle. The available evidence clearly indicates that there are serious
potential hazards associated with the use of the CaMV promoter. All GM
crops and products containing the CaMV promoter should therefore be
withdrawn both from commercial use and from field trials unless and until
they can be shown to be safe.
Hodgson, J. (2000). Nature Biotechnology 18, 13.
Institute of Science in Society website: <www.i-sis.org.uk>
Ho, M.W., Ryan, A. and Cummins, J. (1999). Microbial Ecology in
Health and Disease , in press, and available in electronic form
Wintermantel, W. and Schoelz, J. (1996). Virology 223, 156-64
Ewen, S.W.B. and Pusztai, A. (1999). The Lancet 354,
See for example, Rekvig, O.P., et al (1992). Scand. J.
Immunol. 36, 487-95.
Structural instability of artificial vectors is a text-book topic.
See Old, R.W. and Primrose, S.B. (1994). Principles of gene
manipulation, 5th ed., Blackwell, Oxford.
Covey, S., et al (1990). Proc. Nat. Acad. Sci. USA
Maiss, E., et al (1992). J. Gen. Virol. 73, 709-13;
Meyer, M and Dessens, J. (1997). J. Gen. Viol. 78, 147-51.