Nucleic Acids Research, Vol 26, Issue 15 3591-3598, Copyright © 1998 by Oxford University Press
MA Marchetti, C Tschudi, E Silva and E Ullu
To further our understanding of the structural and functional organization
of the Trypanosoma brucei genome, we have searched for and analyzed sites
in the genome where Pol II transcription units meet Pol III genes. Physical
and transcriptional maps of cosmid clones spanning the Pol III-transcribed
U2 small nuclear RNA (snRNA) and U3 snRNA/7SL RNA gene loci demonstrated
that single-copy Pol II genes are closely associated with Pol
III-transcribed genes, being separated from each other by 0.6-3 kb. At the
U3/7SL transcriptional domain, two Pol II transcription units converged
from either side of the chromosome towards the Pol III genes, suggesting
that at least for the chromosome containing the U3 snRNA and 7SL RNA genes,
there exist two distinct initiation sites for Pol II. Furthermore, in all
cases the Pol III genes hallmark the end of Pol II transcription units,
suggesting perhaps a functional role for this genetic arrangement. Lastly,
we asked whether the environment within a Pol III transcriptional domain
allowed expression of pre-mRNA. To test this we inserted a CAT gene
cassette, seemingly promoterless but endowed with pre-mRNA processing
signals, in the chromosome between the U3 snRNA and 7SL RNA genes.
Interestingly, abundant CAT mRNA was produced suggesting that the Pol III
genes in the immediate vicinity did not prevent access of presumably Pol II
to the CAT gene cassette. We propose that either CAT mRNA is synthesized by
Pol II run-through transcription or by Pol II initiationupstream from the
CAT gene.
ARTICLES
Physical and transcriptional analysis of the Trypanosoma brucei genome reveals a typical eukaryotic arrangement with close interspersionof RNA polymerase II- and III-transcribed genes
Department of Internal Medicine and Department of Cell Biology, Yale University School of Medicine,333 Cedar Street, New Haven, CT 06520- 2088, USA.
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