title: Promoter occupancy of the basal class I transcription factor A differs strongly between active and silent VSG expression sites in Trypanosoma brucei.
abstract: Monoallelic expression within a gene family is found in pathogens exhibiting antigenic variation and in mammalian olfactory neurons. Trypanosoma brucei, a lethal parasite living in the human bloodstream, expresses variant surface glycoprotein (VSG) from 1 of 15 bloodstream expression sites (BESs) by virtue of a multifunctional RNA polymerase I. The active BES is transcribed in an extranucleolar compartment termed the expression site body (ESB), whereas silent BESs, located elsewhere within the nucleus, are repressed epigenetically. The regulatory mechanisms, however, are poorly understood. Here we show that two essential subunits of the basal class I transcription factor A (CITFA) predominantly occupied the promoter of the active BES relative to that of a silent BES, a phenotype that was maintained after switching BESs in situ. In these experiments, high promoter occupancy of CITFA was coupled to high levels of both promoter-proximal RNA abundance and RNA polymerase I occupancy. Accordingly, fluorescently tagged CITFA-7 was concentrated in the nucleolus and the ESB. Because a ChIP-seq analysis found that along the entire BES, CITFA-7 is specifically enriched only at the promoter, our data strongly indicate that monoallelic BES transcription is activated by a mechanism that functions at the level of transcription initiation.
authors.affiliation: Department of Genetics and Developmental Biology, University of Connecticut Health Center, Farmington, CT 06030, USA and Research Center for Infectious Diseases, University of Würzburg, 97080 Würzburg, Germany.
authors.forename: Tu N
authors.initials: TN
authors.lastname: Nguyen
authors.forename: Laura S M
authors.initials: LS
authors.lastname: Müller
authors.forename: Sung Hee
authors.initials: SH
authors.lastname: Park
authors.forename: T Nicolai
authors.initials: TN
authors.lastname: Siegel
authors.forename: Arthur
authors.initials: A
authors.lastname: Günzl
mesh_terms: D002466:Cell Nucleolus / Q000737:chemistry; D020868:Gene Silencing; D020459:Genes, rRNA; D011401:Promoter Regions, Genetic; D015800:Protozoan Proteins / Q000032:analysis / Q000378:metabolism*; D014157:Transcription Factors / Q000032:analysis / Q000378:metabolism*; D061785:Transcription Initiation, Genetic; D015533:Transcriptional Activation; D014346:Trypanosoma brucei brucei / Q000235:genetics*; D014643:Variant Surface Glycoproteins, Trypanosoma / Q000235:genetics*
chemical_list: D015800:Protozoan Proteins; D014157:Transcription Factors; D014643:Variant Surface Glycoproteins, Trypanosoma