Monoallelically-expressed Noncoding RNAs form nucleolar territories on NOR-containing chromosomes and regulate rRNA expression

Out of the several hundred copies of rRNA genes that are arranged in the nucleolar organizing regions (NOR) of the five human acrocentric chromosomes, ∼50% remain transcriptionally inactive. NOR-associated sequences and epigenetic modifications contribute to differential expression of rRNAs. However, the mechanism(s), controlling the dosage of active versus inactive rRNA genes in mammals is yet to be determined. We have discovered a family of ncRNAs, SNULs ( <underline>S</underline> ingle <underline>NU</underline> cleolus <underline>L</underline> ocalized RNA), which form constrained sub-nucleolar territories on individual NORs and influences rRNA expression. Individual members of the SNULs monoallelically associate with specific NOR-containing chromosome. SNULs share sequence similarity to pre-rRNA and localize in the sub-nucleolar compartment with pre-rRNA. Finally, SNULs control rRNA expression by influencing pre-rRNA sorting to the DFC compartment and pre-rRNA processing. Our study discovered a novel class of ncRNAs that by forming constrained nucleolar territories on individual NORs contribute to rRNA expression.