The BLM helicase is a DNA repair protein mutated in the hereditary condition Bloom’s syndrome (BS). BLM is best known for its roles in regulating homologous recombination-mediated DNA repair and telomere maintenance. A more limited body of work suggests that BLM regulates stability of the nucleolar ribosomal DNA. Previous work in our laboratory suggested that BLM might function in nucleolar ribosome biogenesis.
In the present study, we investigated a role for BLM in nucleolar RNA polymerase I-mediated ribosomal RNA (rRNA) transcription. We determined that the nucleolar localization of BLM is sensitive to inhibition of RNA polymerase I-mediated transcription by actinomycin D and 4-nitroquinoline-1-oxide, drugs that inhibit RNA polymerase I. We demonstrated that BLM facilitates 45S rRNA transcription using pulse-chase and nuclear run-on techniques and identified RNA polymerase I and DNA topoisomerase I as nucleolar protein partners. DNA topoisomerase I directly interacts with BLM, an interaction mediated predominantly by the C-terminus of BLM. The interaction of BLM with DNA topoisomerase I is functionally significant as it stimulates BLM unwinding activity in vitro using a RNA:DNA hybrid oligomer substrate that models rRNA transcription from rDNA. We established the biochemical requirement of a 3’ overhang of single-stranded DNA for BLM to unwind nucleolar-relevant RNA:DNA hybrids. Additionally, we discovered a physical interaction between BLM and the nucleolar protein nucleophosmin (NPM) both in nucleoli and nucleoplasm, suggesting a mechanism by which nucleolar trafficking of BLM may be mediated. Overall, this work demonstrates that BLM functions in a pathway of nucleolar ribosome biogenesis and suggests a mechanism by which it may do so. These findings may impact our ability to inhibit or promote cell growth in appropriate clinical settings.