Proofreading in Transcription?
RNA Fills In Another Missing Piece
The
process of proofreading in replication has long been understood, but until
recently no such ability had been shown in transcription. To some extent,
proofreading was not considered as necessary. An analogy would be the
difference between a misprint in a cookbook and misreading the cookbook. In the
first case, if the cookbook itself were wrong (DNA), then every attempt to read
it would lead to a poor baking attempt. In the latter case, a person who
misread the cookbook would make the mistake only once, analogous to making a
poor RNA copy one time. Although there could be ramifications to making an
incorrect RNA and/or protein, the consequences would be much worse if the DNA
itself was wrong.
However,
with our current model of the origins of life being based on RNA that had both
replicative and catalytic ability, the puzzle was missing a piece. If RNA were
the original genetic molecule, then the RNA would have to be replicated
faithfully, and some proofreading ability would be necessary. With DNA proofreading,
separate proteins do the proofreading, but for the RNA-world hypothesis to
hold, RNA would need to have this ability without any protein, which had not
evolved yet. Work by Zenkin et al. cited in the bibliography has shown that RNA
can indeed catalyze its own proofreading, filling in our missing piece. They
demonstrated that a misincor-porated nucleotide could bend back on the previous
nucleotide and, in the presence of Mg2+ and
water, catalyze its own cleavage of the phosphodiester bond, as shown in the
figure.
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