Introduction Analysis of Genomes by Reassociation Experiments Organization of Single-copy Sequences Evolution of Repeated Sequences in Cereals Estimating the Number of Expressed Genes Chloroplast Genome Organization Mitochondrial Genome Organization RNA Editing |
RNA EditingThe Central Dogma of Molecular Genetics states that the information that is found in DNA is used to produce mRNA molecules that are instrumental in the production of proteins. Therefore, the information flows directly from DNA to protein, via the RNA intermediate molecule. Recently it has been discovered that the information that is contained in the DNA is not always found in the RNA products used to make proteins.It has now been demonstrated that mitochondria and chloroplast contain the biochemcial machinery to alter the sequence of the final transcription product. This process is called RNA editing. This process was identified in the following manner. Sequence analysis of a number of cytochrome c oxidase subunit II genes from non-plant species revealed that a tryptophan residue was invariant at several locations in the final protein product. But sequence analysis of this gene in several plant species revealed arginine at those positions. This amino acid change would cause a radical alteration in protein structure because an acidic amino acid would replace a neutral, hydrophobic amino acid. Position 1 Plants: GLU ILE LEU ARG THR ILE PHE PRO Bovine: GLU THR ILE TRP THR ILE PHE PRO Position 2 Plants: GLN TRP TYR ARG THR TYR Bovine: GLN TRP TYR TRP SER TYR
Since a single base pair change in the codons for the two amino acids could generate this
change (CGG for UGG), it was suggested that CGG encoded for tryptophan and not arginine in
plant mitochondria. (This is the only change in codon usage that has been suggested for plants
and has been postulated for several other genes as well.) But this change in codon usage was not
universal, that is some CGG codons actually specified arginine in the final protein product.
Furthermore, no amino acyl tRNA that recognized CGG was found to be
charged with tryptophan, a prerequisite if this codon specification
was actually real.
The solution to this dilemma was found by sequencing the mRNA products for cytochrome
oxidase subunit II genes. It was found that in the mRNA the cytosine residue had been
changed (edited) to uridine at the sequence location where the invariant tryptophan residue is
found. This changed the codon at that location to UGG which is recognized by a
tRNA that carries the amino acid tryptophan. An analysis of three other plant mitochondrial
genes where the same altered codon usage was predicted suggested that mRNA editing was also
occurring at the codon and that a cytosine residue was edited to
uridine. This editing process has also been
detected in protozoa and it remains to be determined if RNA editing is a widespread function in
mitochondria. A final point that this editing function highlights is that the sequence that is found
in the DNA is not entirely and faithfully represented in the final protein product.
Specific Features of RNA Editing
Copyright © 1998. Phillip McClean
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