Science in Society Archive

A Brief History of the CaMV 35S Promoter Controversy

The cauliflower mosaic virus (CaMV) 35S promoter has been widely used in genetically modified (GM) plants before some of its worrying features came to light. The most serious of these is that it has a recombination hotspot, where it tends to fragment and join up with other double-stranded DNA. The definitive evidence for this came in the late 1990s [1,2], although this was suspected much earlier. Dr. Mae-Wan Ho

Evidence emerging since the early 1990s already cast major doubts over the safety of viral genes that were being incorporated into GM crops to make crops resistant to viral attack. Many of the viral genes tended to recombine with other viruses to generate new, infectious viruses. Prof. Joe Cummins of the University of Western Ontario, Canada, was almost a lone voice in objecting to the CaMV 35S promoter on account of evidence suggesting that there was a recombination hotspot associated with the promoter.

We wrote a review on the safety implications of this and other findings concerning the CaMV 35S promoter [3]. We pointed out that its recombination hotspot is 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, and recombination between viral transgenes and infecting viruses has been demonstrated in the laboratory. In addition, 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.

These 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 were especially relevant in the light of the report 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)" [4]. We called for all transgenic crops containing the CaMV 35S promoter to be immediately withdrawn from release.

At least nine criticisms of our paper appeared on the internet even before our paper appeared in print, the most outrageous and abusive of which were incorporated into an 'analysis' written by an editor of Nature biotechnology under "Business and regulatory news" [5]. That 'analysis', concocted entirely of hearsay and opinions, contained such defamatory, libelous statements that the journal had to give us a right to reply when challenged. Our reply was published [6], along with the journalist's 'apology' that he had failed to cite our rebuttal, but he attacked us yet again. This time, Nature biotechnology refused to let us reply.

All of the substantive scientific criticisms eventually turned up in a paper published in the journal where our original paper appeared [7]. The main criticisms boil down to the following.

First, people have been eating the virus in infected cabbages and cauliflower for many years without harm. Second, plants are already loaded with pararetroviral sequences, not unlike CaMV, so why should there be any risks? We rebutted these and other points thoroughly in a paper that was longer than the original [8], and there have been no replies to that paper ever since. The biotech proponents have taken to selectively citing only our first paper and critiques thereof [9].

In answer to the first criticism, that we have been eating CaMV in infected cabbages apparently without harm, we pointed out, first, that intact virus is not the same as naked viral genomes. Naked viral genomes have been found to give full-blown infections, when introduced into non-host species that are not susceptible to the intact virus. Second, the promoter in the viral genome is not the same as the isolated promoter joined to other DNA sequence. The fact that the virus has been infecting cruciferae plant cells for at least millions of years without its promoter ending up in the plant genome, tells us that the natural life cycle of the CaMV is highly regulated and adapted to the biology of its host. The viral genome of pararetroviruses, such as CaMV, generally does not integrate into host genome to complete their lifecycle; and viral replication takes place in the cytoplasm.

But when the isolated promoter is joined to other DNA sequences and artificially integrated into the plant genome, an entirely new genetic and evolutionary context is created where the natural regulatory mechanisms will no longer apply.

Proviral sequences are present in all genomes. As all viral promoters are modular, and have at least one module - the TATA box - in common, and possibly a lot more, it is not inconceivable that the 35S promoter in transgenic constructs can reactivate dormant viruses or generate new viruses by recombination. Thus, when the CaMV 35S promoter was artificially joined to the cDNAs of a wide range of viral genomes, infectious viruses were produced. There is also evidence that proviral sequence in the genome can be reactivated by various environmental stimuli, one of which may be the integration of foreign DNA.

The fact that plants are "loaded" with proviral and other 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 [10]. 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.

Not only have our critics failed to reply, they persist in perpetrating the falsehood that the CaMV 35S promoter is only active in plants [11] when it is generally known that the promoter is active in a wide range of species including bacteria, yeast and algae.

In the course of the debate, we discovered even more damning evidence in literature dating back to 1989. The CaMV 35S promoter is active in frog eggs and human cell systems [12].

When we first called for transgenic crops containing the CaMV 35S promoter to be withdrawn, one of our fiercest critics was the head of the research group in John Innes Center that discovered the recombination hotspot associated with the CaMV 35S promoter. Two years later, the same group finally recommended that the promoter should be phased out on account of the structural instability it causes [13]. But structural instability would indeed make it prone to horizontal gene transfer and recombination, as we have pointed out.

Last November, a paper published in Nature [14] reported that Mexican maize landraces growing in remote regions have been contaminated by transgenes. The paper is now at the centre of a storm whipped up by scientists supporting biotech, who are criticising it for 'poor methodology'. This led to a retraction by Nature, which is unprecedented for a paper that has not been proved wrong or fraudulent. The critics are not contesting the fact that transgenic contamination has occurred and that the CaMV 35S promoter has been found in the landraces. In fact, the Berkeley researchers who wrote the original report were able to present new data firming up their conclusion that transgenic contamination in the form of CaMV 35S promoter had occurred. What their critics are contesting is the form in which the promoter has entered the landraces' genomes [15].

The researchers claimed to have found that the promoter in the landraces' genomes is linked, not to the original transgenes, but to a variety of other DNA sequences. This is as though the promoter has broken off and joined up at random, or as stated by one of the critiques, "fragmenting and promiscuously scattering throughout genomes". This, the critics are strenuously denying.

But that is far from unexpected, in view of the 'recombination hotspot' associated with the CaMV 35S promoter, discovered years after transgenic crops containing the promoter has been widely released. It is of interest that when Monsanto's Roundup Ready soya was subjected to 'event specific' molecular analysis [16], the transgenic insert was found scrambled, as was the host genome at the site of insertion. And a DNA fragment of more than 500 basepairs of unknown origin is also present. None of that was reported in the company's original application submitted for commercial approval.

The companies must now be asked to provide both past and present molecular data on their transgenic lines, and submit samples for independent analysis. Meanwhile, there is all the more reason to stop releasing transgenic crops containing the CaMV 35S promoter.

Article first published 05/10/09


  1. Kohli A, Griffiths S, Palacios N, Twyman RM, Vain P, Laurie DA, 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. The Plant Journal 1999, 17: 591-601.
  2. Kumpatla SP. and Hall TC. Organizational complexity of a rice transgenic locus susceptible to methylation-based silencing. IUBMB Life 1999: 48: 459-467.
  3. Ho MW, Ryan A, Cummins, J. Cauliflower Mosaic Viral Promoter - A Recipe for Disaster? Microbial Ecology in Health and Disease 1999, 11: 194-197.
  4. Ewen S, Pusztai A. Effect of Diets Containing Genetically Modified Potatoes Expressing Galanthus nivalis Lectin on Rat Small Intestine. The Lancet 1999, 354: 1353-1354.
  5. Hodgson J. Scientists avert new GMO crisis. Nature Biotechnology 2000, 18, 13.
  6. Cummins J, Ho MW and Ryan A. Hazardous CaMV promoter? Nature Biotechnology 2000, 18, 363.
  7. Hull R, Covey SN and Dale P. Genetically modified plants and the 35S promoter: assessing the risks and enhancing the debate. Microbial Ecology in Health and Disease 2000, 12, 1-5.
  8. Ho MW, Ryan A and Cummins J. Hazards of transgenic plants with the cauliflower mosaic viral promoter. Microbial Ecology in Health and Disease 2000, 12, 6-11.
  9. Public Hearing for the proposed addition of CHARDON LL to the UK National List, Oral Submission made on behalf of Aventis Cropscience UK Ltd by Herbert Smith, delivered to the public hearing on 29th May 2002 (Document AV 59).
  10. "GM crops may face genetic melt down" by Joe Cummins ISIS News 2001, 9/10
  11. "GM maize approved on bad science" by Mae-Wan Ho Science in Society 15, 10-25..
  12. Ho MW, Ryan A and Cummins J. CaMV 35S promoter fragmentation hotspot confirmed, and it is active in animals. Microbial Ecology in Health and Disease 2000, 12, 189.
  13. Christou P, Kohli A, Stoger E, Twyman RM, Agrawal P, Gu X. Xiong J, Wegel E, Keen D, Tuck H, Wright M, Abranches R and Shaw P. Transgenic plants: a tool for fundamental genomics research. John Innes Centre & Sainsbury Laboratory Annual Report 1999/2000, p. 29. See "Top research centre admits GM failure" Transgenic Instability, I-SIS Reprints, I-SIS Publications, London, March 2002.
  14. Quist, D. & Chapela, I.H. Transgenic DNA introgressed into traditional maize landraces in Oaxaca, Mexico. Nature 2001, 414, 541-3.
  15. See "Astonishing denial of transgenic contamination", by Mae-Wan Ho, to appear in Science in Society 2002, 15, Institute of Science in Society, London
  16. Windels P, Taverniers I, Depicker A, Van Bockstaele E and De Loose M. Characterisation of the Roundup Ready soybean insert. Eur Food Res Technol DOI 10.1007/ s002170100336, © Springer-Verlag, 2001; see also "Scrambled genome of Roundup Ready soya" by Mae-Wan Ho, ISIS News 2001, 9/10, Institute of Science in Society, London

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Liz Chafer Comment left 6th October 2009 06:06:16
All the wind turbines installed in Germany have had little impact on Co2 emissions in that country due to the intermittency of wind . The same is the case for Denmark cf. the CEPOS report sept 2009 In many countries including USA Canada France and the UK there is immense pressure to install industrial wind turbines often within a mile of houses affecting the health of those living in the vicinity. Even where I live the load factor will only be 15% but lack of wind doesn't seem to present a problem to the wind industry promoters. Intermittency is a serious problem until there is a means of stocking the energy produced.There are other forms of renewable energy that should be developed in preference to that of wind - wave power for example.

Mark Russell Comment left 24th October 2009 02:02:38
Those are big, sweeping statements about nuclear ... "the nuclear black hole in terms of cost, safety and sustainability." All (except maybe sustainability in the grand scheme) are nothing more than populist fears, and not based on fact or reality. Wind power is positively medival by comparison no matter how you dress it up. Once the planet is out of all other forms of energy wind power may make sense (and by implication it is last on my list of desirable technologies). By that time the place will look like "planet of the apes" anyway so wind power will fit right in.

Mae-Wan Ho Comment left 23rd October 2009 06:06:49
All you people against wind and pro-nuclear really want to come down to earth and look at the nuclear black hole in terms of cost, safety and unsustainability. We are not for big wind farms. Cheap affordable small wind turbines are here! Read our complete report and get a full picture. Join the dots and join the future.

Mark Russell Comment left 23rd October 2009 06:06:18
Once the "storage solution" is worked out, why would you fill that storage with energy produced by industrial wind turbines when you could fill it with energy produced by nuclear power at a fraction of the cost, and without the massive eyesore that are wind farms? A storage solution notwithstanding, the cost of wind power in general will cause countries that have committed to wind power losing almost all of their manufacturing base to countries that can supply cheap, reliable energy, whether clean or dirty (manufacturers don't really care).