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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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