Cosupression
What happens when a little genetic engineering goes a little too far?
[Reference: "An RNA-Based Informarion Superhighway in Plants" (Richard A.
Jorgensen, Ross G. Atkinson, Richard L. S. Forster, William J. Lucas) from
Science Vol. 279, March 6, 1998, pg 1486- 1487.]
Genetic engineers try to improve plants. They make grains with
increased protein content, fruits and vegetables with enhanced nutritional
value, and flowers with deeper colors. However, in attempting to create
such plants, by overexpressing the plant's own proteins, geneticists
have often inadvertently caused to opposite to occur. "Instead of producing
large quantites of new proteins, high-expressing transgenes introduced into the
plant can actually inhibit the expression of the plant's own genes by triggering
sequence specific destruction of similar transcripts. Thus, the transgene ends
up silencing both its own expression and that of similar endogenous genes when
the concentration of transgene transcript (mRNA) becomes too high in the
cytoplasm. This unintended "cosupression" can nonetheless be harnessed by
genetic engineers-- to eliminate unwanted gene expression, for example-- and is
used by the plant itself to inhibit protein synthesis by invading viruses." (1486)
Example: A plant leaf expressing the green fluorescent protein (GFP)
from jellyfish is infiltrated with an Agrobacterium tumefaciens strain
carrying a GFP gene within its transfer DNA (T-DNA). This T-DNA integrates into
the nuclear genome of cells in the exposed leaf. Although the bacterium and the
T-DNA are restricted to the infiltrated leaf, GFP expression is silenced through
the plant.
COSUPRESSION
Information transfer through the plant. (A) Long
distance (phloem) transmisson of the cosupression state. (B) Model
for the plasmodesmal trafficking and propagation of an RNA surveillance
signal within tissues expressing the transgene. STP, surveillance translocation
protein (facillitates cRNA cell-to-cell and long-distance trafficking); RdRNP,
RNA polymerase; dsRNase, double-stranded ribonuclease. (C) cRNA-
STP complex entering from the companion cell (CC) to the sieve element (SE) of
the phloem (ER, endoplamic reticulum).
(picture and information from Science vol. 279, March 6, 1998, pg. 1486)
Email: mageelb@miavx1.muohio.edu
