title: Collaborating functions of BLM and DNA topoisomerase I in regulating human rDNA transcription.
abstract: Bloom's syndrome (BS) is an inherited disorder caused by loss of function of the recQ-like BLM helicase. It is characterized clinically by severe growth retardation and cancer predisposition. BLM localizes to PML nuclear bodies and to the nucleolus; its deficiency results in increased intra- and inter-chromosomal recombination, including hyper-recombination of rDNA repeats. Our previous work has shown that BLM facilitates RNA polymerase I-mediated rRNA transcription in the nucleolus (Grierson et al., 2012 [18]). This study uses protein co-immunoprecipitation and in vitro transcription/translation (IVTT) to identify a direct interaction of DNA topoisomerase I with the C-terminus of BLM in the nucleolus. In vitro helicase assays demonstrate that DNA topoisomerase I stimulates BLM helicase activity on a nucleolar-relevant RNA:DNA hybrid, but has an insignificant effect on BLM helicase activity on a control DNA:DNA duplex substrate. Reciprocally, BLM enhances the DNA relaxation activity of DNA topoisomerase I on supercoiled DNA substrates. Our study suggests that BLM and DNA topoisomerase I function coordinately to modulate RNA:DNA hybrid formation as well as relaxation of DNA supercoils in the context of nucleolar transcription.
authors.affiliation: Department of Microbiology, Immunology and Medical Genetics, The Ohio State University College of Medicine, Columbus, OH 43210, USA.
authors.forename: Patrick M
authors.initials: PM
authors.lastname: Grierson
authors.affiliation: Department of Microbiology, Immunology and Medical Genetics, The Ohio State University College of Medicine, Columbus, OH 43210, USA. Electronic address: samir.acharya@osumc.edu.
authors.forename: Samir
authors.initials: S
authors.lastname: Acharya
authors.affiliation: Department of Microbiology, Immunology and Medical Genetics, The Ohio State University College of Medicine, Columbus, OH 43210, USA.
authors.forename: Joanna
authors.initials: J
authors.lastname: Groden
mesh_terms: D001816:Bloom Syndrome / Q000201:enzymology / Q000235:genetics / Q000378:metabolism; D002460:Cell Line; D045744:Cell Line, Tumor; D002466:Cell Nucleolus / Q000201:enzymology / Q000235:genetics / Q000378:metabolism; D004264:DNA Topoisomerases, Type I / Q000235:genetics* / Q000378:metabolism*; D004275:DNA, Ribosomal / Q000235:genetics* / Q000378:metabolism; D057809:HEK293 Cells; D006801:Humans; D061986:MCF-7 Cells; D053484:RecQ Helicases / Q000235:genetics* / Q000378:metabolism*; D014158:Transcription, Genetic
chemical_list: D004275:DNA, Ribosomal; C097151:Bloom syndrome protein; D053484:RecQ Helicases; D004264:DNA Topoisomerases, Type I; C528909:TOP1 protein, human