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* Department of Chemistry, University of Minnesota, Minneapolis, MN
55455; and
Contributed by Paul R. Schimmel, July 16, 1997
Oligonucleotides that recapitulate the acceptor stems of tRNAs are
substrates for aminoacylation by many tRNA synthetases in
vitro, even though these substrates are missing the anticodon trinucleotides of the genetic code. In the case of tRNAAla
a single acceptor stem G·U base pair at position 3·70 is
essential, based on experiments where the wobble pair has been replaced
by alternatives such as I·U, G·C, and A·U, among others.
These experiments led to the conclusion that the minor-groove free
2-amino group (of guanosine) of the G·U wobble pair is essential
for charging. Moreover, alanine-inserting tRNAs (amber suppressors)
that replace G·U with mismatches such as G·A and C·A are
partially active in vivo and can support growth of an
Escherichia coli tRNAAla knockout strain,
leading to the hypothesis that a helix irregularity and nucleotide
functionalities are important for recognition. Herein we investigate
the charging in vitro of oligonucleotide and full-length
tRNA substrates that contain mismatches at the position of the G·U
pair. Although most of these substrates have undetectable activity,
G·A and C·A variants retain some activity, which is,
nevertheless, reduced by at least 100-fold. Thus, the in
vivo assays are much less sensitive to large changes in
aminoacylation kinetic efficiency of 3·70 variants than is the
in vitro assay system. Although these functional data do
not clarify all of the details, it is now clear that specific atomic
groups are substantially more important in determining kinetic
efficiency than is a helical distortion. By implication, the activity
of mutant tRNAs measured in the in vivo assays appears
to be more dependent on factors other than aminoacylation kinetic
efficiency.
Proc. Natl. Acad. Sci. USA
Vol. 94,
pp. 10150-10154,
September 1997
Biochemistry
Specific atomic groups and RNA helix geometry in acceptor stem
recognition by a tRNA synthetase
, and
Department of Biology, Massachusetts Institute of
Technology, Cambridge, MA 02139
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