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This means that constructs may be prepared rapidly in the bacteria and delivered into

yeast for expression studies.

The bacteriumAgrobacterium tumefaciensinfects plants that have been damaged

near soil level, and this infection is often followed by the formation of plant tumours

in the vicinity of the infected region. It is now known thatA. tumefacienscontains a

plasmid called the Ti plasmid, part of which is transferred into the nuclei of plant cells

which are infected by the bacterium. Once in the nucleus, this DNA is maintained by

integrating with the chromosomal DNA. The integrated DNA carries genes for the

synthesis of opines (which are metabolised by the bacteria but not by the plants) and

for tumour induction (hence ‘Ti’). DNA inserted into the correct region of the Ti

plasmid will be transferred to infected plant cells, and in this way it has been possible

to clone and express foreign genes in plants (Fig. 6.25). This is an essential prerequisite

for the genetic engineering of crops.

6.3.8 Delivery of vectors into eukaryotes

Following the production of a recombinant molecule, the so-calledconstructsare

subsequently introduced into cells to enable it to be replicated a large number of times

as the cells replicate. Initial recombinant DNA experiments were performed in bacterial

Ti plasmid

Agropine

synthesis

T DNA

pTiCch5

Octapine

catabolism

Agropine

catabolism

ori

vir genes

Transform

plant cell

Plant cell

Nucleus

Integration of Ti DNA

with plant genome

Plate onto agar

Transformed callus

Transfer to medium + hormones

Transformed shoots

Transformed plants

ocs arc

tra

Fig. 6.25Scheme for cloning in plant cells using the Ti plasmid.

222 Recombinant DNA and genetic analysis