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Notes:
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If the enzymes responsible for biosynthesis of a given
amino acid are repressed and the cognate amino acid
pool suddenly depleted, then derepression of these
enzymes and replenishment of the pool would be
problematic, if the enzymes were largely composed of
the cognate amino acid. In the proverbial ‘Catch 22’,
cells would lack the necessary enzymes to make the
amino acid, and they would lack the necessary amino
acid to make the needed enzymes. Based on this
scenario, we hypothesize that evolution would lead to
the selection of amino acid biosynthetic enzymes that
have a relatively low content of their cognate amino
acid. We call this the ‘cognate bias hypothesis’. Here
we test several implications of this hypothesis
directly using data from the proteome of Escherichia
coli. Several lines of evidence show that low cognate
bias is evident in 15 of the 20 amino acid biosynthetic
pathways. Comparison with closely related Salmonella
typhimurium shows similar results. Comparison
with more distantly related Bacillus subtilis shows
general similarities as well as significant differences
in the detailed profiles of cognate bias. Thus, selection
for low cognate bias plays a significant role in
shaping the amino acid composition for a large class
of cellular proteins. |
