Cis-interactions between non-coding ribosomal spacers dependent on RNAP-II separate RNAP-I and RNAP-III transcription domains.

Ribosome biogenesis requires transcription of structural RNAs. In budding yeast, ribosomal units contain both 35S and 5S RNA genes separated by intergenic spacer sequences (IGS) that are transcribed by RNAP-II. IGS transcripts cause instability by promoting unequal sister chromatid recombination between repeats and are thus rapidly degraded by the exosome. Whether RNAP-II within IGS regions plays any functional role is unknown. Here we demonstrate that the bulk of RNAP-II bound to IGS sites is blocked for elongation and hence remains in a poised or stalled configuration. We describe a novel role for these stalled RNAP-II complexes in the formation of cis-interactions between the IGS of rDNA. We show that this function separates 35S and 5S RNA genes into polymerase-specific chromatin loops and demonstrate that removal of stalled RNAP-II complexes causes displacement of RNAP-III from the 5S gene region and transcriptional downregulation of 5S rRNA by spreading of RNAP-I. We conclude that stalled RNAP-II plays an active role in the cis-organisation of ribosomal repeats providing domains of polymerase specificity in the nucleolar transcription environment.