ISWI chromatin remodelers are highly conserved in
eukaryotes and are important for the assembly and spacing of
nucleosomes, thereby controlling transcription initiation and
elongation. ISWI is typically associated with different subunits,
forming specialized complexes with discrete functions. In the unicellular
parasite Trypanosoma brucei, which causes African sleeping
sickness, TbISWI down-regulates RNA polymerase I (Pol
I)-transcribed variant surface glycoprotein (VSG) gene expression
sites (ESs), which are monoallelically expressed. Here, we use tandem
affinity purification to determine the interacting partners of
TbISWI. We identify three proteins that do not show significant
homology with known ISWI-associated partners. Surprisingly, one
of these is nucleoplasmin-like protein (NLP), which we had previously
shown to play a rolein ES control. In addition, weidentify two
novel ISWI partners, regulator of chromosome condensation
1-like protein (RCCP) and phenylalanine/tyrosine-rich protein
(FYRP), both containing proteinmotifs typically found on chromatin
proteins. Knockdown of RCCP or FYRP in bloodstream form
T. brucei results in derepression of silent variant surface glycoprotein
ESs, as had previously been shown for TbISWI and NLP. All
four proteins are expressed and interact with each other in both
major life cycle stages and show similar distributions at Pol I-transcribed
loci. They are also found at Pol II strand switch regions as
determined with ChIP. ISWI, NLP, RCCP, and FYRP therefore
appear to form a single major ISWI complex in T. brucei (TbIC).
This reduced complexity of ISWI regulation and the presence of
novel ISWI partners highlights the early divergence of trypanosomes
in evolution.