MadSci Network: Genetics

Re: Can I purchase living Agrobacterium tumefacien with desired genes?

Date: Thu May 27 13:38:42 1999
Posted By: Mark Sullivan, Staff, Molecular and Microbilogy, Fred Hutchinson Cancer Res. Center
Area of science: Genetics
ID: 926962576.Ge

     The ability to transfer genes from one species to another is a very 
common practice in molecular biology labs.  What you are refering to is a 
gene called "luciferase" which is a protein that causes 
chemiluminescence (glowing) when a certain substrate is added.  This is a 
naturally-occurring protein found in fireflies.  There are several types of 
genes that scientists utilize as reporter genes.  When we try to put DNA 
from one type of organism or source into another organism, we need to know 
not only that it got into the target cell's DNA, but also that it's being 
activated.  The use of a simple gene that can report to us the presence of 
the transferred DNA by flourescence or some color change is very helpful.  
To make a plant glow in the dark is interesting and fun to see, but is not 
the ultimate purpose of the experimentor.  
     Let's look at a possible research scenario.  Perhaps the researcher 
wanted to see if human antibodies could be produced in plants for medical 
purposes.  DNA containing genes for a specific antibody and the luciferase 
reporter are transferred to the plant cells by use of some kind of vector, 
i.e. some kind of transport or shuttle to deliver the desired DNA to the 
target.  Viruses can be used as vectors, and also in the case of plants, a 
bacterium called Agrobacterium tumefaciens.  This bacteria is able to 
transfer some of its genetic material into plant cells so that the plant 
will make some of the things it needs to survive, and also causes what is 
known as crown gall.  Scientists can trick the bacterium into delivering 
the genes that they are interested in to the plant. It is relatively easy 
to replace the normal bacterial gene that is delivered with a gene we want 
to study by use of recombinant DNA techniques.  So now we have a strain of 
Agrobacterium that carries our gene of interest, and we inoculate a plant 
with that bacterium.  It delivers the luciferase and antibody genes to the 
plant nucleus where hopefully the plant is able to start producing protein 
from both of these new genes.  How do we tell if we are successful?  Well, 
we could just break open the cells and go through lots of long 
purification processes and tests to detect functional antibody made by the 
plants.  But it is a lot easier to see if the DNA is even there by checking 
for luminescence conferred by the luciferase gene.  IF the plant glows when 
we add a certain chemical to the plant, then the DNA we wanted to put in is 
there, and since luciferase is being made then most likely the plant is 
making antibodies as well.  We can test all the plants that glow for 
antibodies and disregard the ones that don't glow.  The luciferase gene 
reported to us that the DNA is there.  If you want to have plants with 
almost every cell containing these genes, you transfer them to plant cells 
at a very early stage of development so when cells divide, the daughter 
cells have all the same genetic material as the parent cell.  Then you can 
have adult plants that may be able to pass these foreign genes on to their 
offspring as well.
     Now, as to whether you can buy Agrobacterium tumefaciens, I don't know 
why one person would want to.  Also, I am not sure, but I don't think you 
can buy Agrobacterium with desired genes.  The genes of interest are 
contained on small circular pieces of DNA called plasmids, which are kind 
of like cassette tapes that you can put into bacteria.  There are all kinds 
of plasmids, and each lab using this technology genetically manipulates 
their own and puts them into the bacteria.  You can certainly find these 
bacteria naturally in trees that have crown gall, those big tumorous-
looking knots in the trunks or branches.  But unless you have a research 
lab with lots of equipment, biological and chemical supplies to work with 
the bacteria,and perform recombinant DNA technology on them, you might as well 
just leave it in the tree.       

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