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Mol. Cell. Biol., Jan 1998, 276-289, Vol 18, No. 1
G Yeung, LM Choi, LC Chao, NJ Park, D Liu, A Jamil and HG Martinson
We mapped the elements that mediate termination of transcription downstream
of the chicken betaH- and betaA-globin gene poly(A) sites. We found no
unique element and no segment of 3'-flanking DNA to be significantly more
effective than any other. When we replaced the native 3'-flanking DNA with
bacterial DNA, it too supported transcription termination. Termination in
the bacterial DNA depended on a functional poly(A) signal, which apparently
compelled termination to occur in the downstream DNA with little regard for
its sequence. We also studied premature termination by poorly processive
polymerases close to the promoter. The rate of premature termination varied
for different DNA sequences. However, the efficiencies of poly(A)-driven
termination and promoter-proximal premature termination varied similarly on
different DNAs, suggesting that poly(A)-driven termination functions by
returning the transcription complex to a form which resembles a prior state
of low processivity. The poly(A)-driven termination described here differs
dramatically from the poly(A)- assisted termination previously described
for the simian virus 40 (SV40) early transcription unit. In the SV40 early
transcription unit, essentially no termination occurs downstream of the
poly(A) site unless a special termination element is present. The
difference between the betaH-globin and SV40 modes of termination is
governed by sequences in the upstream DNA. For maximum efficiency, the
betaH-globin poly(A) signal required the assistance of upstream enhancing
sequences. Moreover, the SV40 early poly(A) signal also drove termination
in betaH- globin style when it was placed in a betaH-globin sequence
context. These studies were facilitated by a rapid, improved method of
run-on transcription analysis, based on the use of a vector containing two
G- free cassettes.
Copyright © 1998, American Society for Microbiology
Poly(A)-driven and poly(A)-assisted termination: two different modes of poly(A)-dependent transcription termination
Department of Chemistry and Biochemistry, University of California at Los Angeles, 90095-1569, USA.
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