Erschienen:
Proceedings of the National Academy of Sciences, 1998
Erschienen in:Proceedings of the National Academy of Sciences
Sprache:
Englisch
DOI:
10.1073/pnas.95.21.12328
ISSN:
0027-8424;
1091-6490
Entstehung:
Anmerkungen:
Beschreibung:
<jats:p>
In
<jats:italic>Trypanosoma brucei</jats:italic>
, transcription by RNA polymerase II and 5′ capping of messenger RNA are uncoupled: a capped spliced leader is trans spliced to every RNA. This decoupling makes it possible to have protein-coding gene transcription driven by RNA polymerase I. Indeed, indirect evidence suggests that the genes for the major surface glycoproteins, variant surface glycoproteins (VSGs) in bloodstream-form trypanosomes, are transcribed by RNA polymerase I. In a single trypanosome, only one
<jats:italic>VSG</jats:italic>
expression site is maximally transcribed at any one time, and it has been speculated that transcription takes place at a unique site within the nucleus, perhaps in the nucleolus. We tested this by using fluorescence
<jats:italic>in situ</jats:italic>
hybridization. With probes that cover about 50 kb of the active
<jats:italic>221</jats:italic>
expression site, we detected nuclear transcripts of this site in a single fluorescent spot, which did not colocalize with the nucleolus. Analysis of marker gene-tagged active expression site DNA by fluorescent DNA
<jats:italic>in situ</jats:italic>
hybridization confirmed the absence of association with the nucleolus. Even an active expression site in which the promoter had been replaced by an rDNA promoter did not colocalize with the nulceolus. As expected, marker genes inserted in the rDNA array predominantly colocalize with the nucleolus, whereas the tubulin gene arrays do not. We conclude that transcription of the active
<jats:italic>VSG</jats:italic>
expression site does not take place in the nucleolus.
</jats:p>