Nucleic Acids Research, Vol 27, Issue 4 1159-1167, Copyright © 1999 by Oxford University Press
LP Ford and J Wilusz
We have previously shown that the presence of a poly(A) tail blocks the
activity of a highly efficient 3'-->5' exonuclease in HeLa extracts.
Similar activities have been implicated in RNA turnover in vivo. It is not
clear, however, what protects poly(A)-non-mRNAs from the action of this
enzyme. A stem-loop structure located at the 3'-end of U11 RNA was required
to protect this transcript from the exonuclease in vitro. Similar 3'
stem-loop structures, or extensive base pairinginvolving the 3'-end, are
present on all mature small stable RNAs. The placement of artificial
stem-loop structures at the 3'-end also protected RNA substrates,
suggesting that RNA structure alone is sufficient to block the initiation
of the exonuclease. The placement of RNA structures at internal positions
of substrate trans-cripts did not affect the activity of the exonuclease or
lead to the accumulation of degradation intermediates. Pol III precursor
transcripts contain short poly(U) tracts rather than structure at their
3'-ends. Terminal poly(U) tracts protected RNA substrates from the
3'-->5' exonuclease in a protein- dependent fashion. Although La protein
is found associated with the terminal U tracts of pol III precursor
transcripts both in vivo and in vitro, La protein was not required for
poly(U) to protect RNA substrates from the 3'-->5' exonuclease. In
summary, these data reveal a variety of ways RNAs have evolved to protect
themselves from this exonuclease.
ARTICLES
3'-Terminal RNA structures and poly(U) tracts inhibit initiation by a 3'-->5' exonuclease in vitro
Department of Microbiology and Molecular Genetics, UMDNJ-New Jersey Medical School, Newark, NJ 07103, USA.
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