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[SANET-MG] cytoplasmic male sterility by RNA editing
January 2, 2007
Prof. Joe Cummins
Cytoplasmic Male Sterile Lines by RNA Editing
Cytoplasmic male sterility is the most commonly employed
form of male sterility employed in making hybrids. Male sterility is
caused by defects in the mitochondrial energy network. Mitochondria
have a chromosome and genes independent of the nuclear genes as do the
chloroplasts. Mitochondrial proteins are made up of a mixture of
proteins specified by mitochondrial genes and proteins transported into
the mitochondria from the cytoplasm from messenger RNA produced in the
nucleus. Cytoplasmic male sterility is caused by defects in proteins
specified by mitochondrial genes concerned with energy production in the
mitochondria.
Plant mitochondrial genes unlike animal mitochondrial genes frequently
are split into introns and exons and the messenger RNAs made from the
mitochondrial genes are often edited after transcription.; Certain
specified C (cytosine bases) are converted to U (uracil bases) by
enzymes that act on the completed messenger RNA changing code word
meaning specifying amino acids , stops or starts.
The system for male sterility developed by Patell et al (1)
for producing hybrid rice consisted disrupting the editing of the nad9
mitochondrial gene (part of the preimary energy generating complex) of
rice. A nuclear gene specifying unedited nad9 messenger RNA to produce
the unedited protein product was introduced into the nuclear genes to
which a mitochondrial transit signal was added along with a ubiquitin
promoter and NOS terminator accompanied by an antibiotic resistance
gene. The protein from the unedited transcript was inserted into the
primary energy complex disrupting the function of the complex and
creating male sterility (death to pollen). The fertility of the male
sterile line was restored by crossing that line with a line bearing a
synthetic anti-sense gene for the unedited nad9 messenger RNA. The
sense and anti-sense messenger RNAs combine to produce a double stranded
RNA that is recognized as if it was a replicating virus and destroyed
by the plants innate virus defense system.
A very similar system for cytoplasmic male sterility and
its restoration was earlier developed for canola (2). In that system
the atp6 unedited gene of the mitochondria was introduced into the
nucleus while the fertility was restored by an edited copy of the atp6
gene in the nucleus. The edited nuclear gene was not as effective in
restoring fertility as was the antisense gne used in the rice system
Cytoplasmic male sterility based on RNA editing is intrinsically safer
than is the using of toxic product such as barnase to ablate (kill)
pollen Barnase toxin and the barnase barstar complex are potentially
allergens as well However, the RNA editing forms are still constructed
using antibiotic resistance genes and they both involve illegitimate
recombination as does the barnase system. Nevertheless, RNA editing is
preferred over barnase ,even though both systems require fuller safety
testing for humans and for the environment.
References
1. Patell,V,Rayapuram,N,Venkataraamiah,Joma,J
and Goswami,S. Process fror generating cytoplasmic male sterile line in
rice and other crops by rna editing. United States Patent Application
20030163856
2. Brown,C. Method for the enhancement of
naturally occurring cytoplasmic male sterility and for restoration of
male fertility and uses thereof in hybrid crop production United States
Patent 2002 6,365,798
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