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Post-doc available

Thu Jan 21 09:23:00 EST 1993

*** The following is a job ad placed on behalf of Dr. Conrad Lichtenstein.  
Please address all enquiries to him at, c.lichtenstein at ic.ac.uk ***

                Imperial College of Science, Technology & Medicine,  London

A POSTDOCTORAL POSITION is available to join an active plant molecular
biology  group to work on a project entitled:

                                                         AN ASSESSMENT OF RISKS"

The position, available from early 1993, is funded by  MAFF for three
years. Applicants with a relevant background in molecular biology should
send a c.v. and names of three referees to Dr Conrad Lichtenstein, 
Department of Biochemistry, Imperial College, London SW7 2AZ (Tel: 071  225
8940; Fax:  071 225 8942). Experience with plant tissue culture and
transformation is preferred but not essential.

This project has its genesis in a serendipitous but remarkable discovery we
made when using antisense RNA to engineer geminivirus  resistance in
transgenic Nicotiana tabacum(tobacco) plants. Our discovery was that
untransformed, ie. non-transgenic, tobacco plants already contain
geminivirus-related DNA sequences integrated into the nuclear genome. 
        We cloned and sequenced portions of this endogenous plant DNA and
found significant homology to the geminivirus, Tomato Golden Mosaic Virus
(TGMV) and also a reduced homology  to other related geminiviruses. 
Sequences homologous to this endogenous tobacco DNA exist in some but not
all, of a dozen other Nicotiana species we have analysed.
        These geminivirus-related plant sequences (hereafter called GRPS)
must have arisen during evolution, suggesting that natural mechanisms exist
for gene transfer between plants and viruses and thus a study of these
events is germane to the assessment of the risks on the release of
genetically modified plants in the agricultural environment.
        The questions to address are: What is the evolutionary origin of
the GRPS? The following possibilities are so far suggested by our data: 
(i) The sequences are of geminiviral origin and arose by integration into
the genome of an ancestor of tobacco (horizontal transmission). If this is
so, than was it a rare event, or do geminiviruses repeatedly transduce
viral DNA to the genomes of their host plants? We may find that such
sequences occur in a variety of quite different plant species (eg, wheat,
maize, beet, cassava and bean) and perhaps show greatest homology to those
geminiviruses which infect them (eg, WDV, MSV, BCTV, ACMV and BGMV
respectively)? (ii)     Alternatively, as viruses presumably evolve from
"runaway" host genes, the viral DNA is of plant origin. The GRPS may be a
member of a family of related plant sequences.  (iii) Perhaps gene flow
between plants and geminviruses goes in both directions and such viruses
are the agents for horizontal gene transfer between plants?
        This study of molecular evolution is exciting as, to my knowledge,
our observation of gene flow between viruses and plants is without
precedent; it is however reminiscent both of the interplay between viral
oncogenes and cellular proto-oncogenes in animals  and of T-DNA
transformation by Agrobacterium rhizogenes during the evolution of the
genus Nicotiana.
        In addition to addressing the evolutionary origin of the GRPS other
questions follow: Do GRPS encode  gene products and if so are they
functional and what is their function?  From the foregoing, though natural
genetic exchange between plant and geminiviral genomes can occur, but how
easily and how;  can we actually demonstrate geminivirus:nuclear genetic
recombination? If so can DNA acquired by a recombinant geminivirus be
transferred by viral infection to another plant? Further, can the naturally
occurring GRPS excise from the plant genome and replicate? 
Dr Conrad Lichtenstein
Department of Biochemistry
Imperial College of Science, Technology & Medicine
Exhibition Road, South Kensington
London SW7 2AZ, England
Tel: (0)71 225 8940, Fax: (0)71 225 8942
Home phone: (0)81 749 8230
email address: c.lichtenste at ic.ac.uk

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