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[Yeast] Re: transcription termination question

yeast at iubio.bio.indiana.edu yeast at iubio.bio.indiana.edu
Thu Apr 28 09:07:12 EST 2005

Yeast messsages end a 3'OH since all nucleotides, including the polyA tail, are
added by normal 5' to 3' RNA synthesis.  The only feature found for actual
cleavage sites that is relatively common is that the cleavage occurs at an NA
sequence, where N is any nucleotide.  There are sequence elements that are
shared.  Most metazoan mRNAs have an AAUAAA located 10-30 nucleotides upstream
of the cleavage site, but in yeast these are much less conserved and less well
characterized.  There is also iimportant information downstream of the cleavage
site (and therefore only in the pre-mRNA), usually U or GU-rich in animal mRNAs
but less well characterized in yeast.

This applies  to pol II transcripts that become mRNAs but not those that become
U-snRNAs, where true termination takes place.  Sequences in the promoters for
U-snRNA genes appear to inform (and alter) the polymerase complex so that it
"knows" it is not producing an mRNA but rather an snRNA, and this allows it to
see its termination signal, which will not function if placed in the context of
a pol II gene producing a polyadenylated mRNA.  microRNAs are produced from
precursors synthesized by pol II and having polyA tails, genereated most likely
by the normal 3' processing mechanism.

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