title: The role of topoisomerase I in suppressing genome instability associated with a highly transcribed guanine-rich sequence is not restricted to preventing RNA:DNA hybrid accumulation.
abstract: Highly transcribed guanine-run containing sequences, in Saccharomyces cerevisiae, become unstable when topoisomerase I (Top1) is disrupted. Topological changes, such as the formation of extended RNA:DNA hybrids or R-loops or non-canonical DNA structures including G-quadruplexes has been proposed as the major underlying cause of the transcription-linked genome instability. Here, we report that R-loop accumulation at a guanine-rich sequence, which is capable of assembling into the four-stranded G4 DNA structure, is dependent on the level and the orientation of transcription. In the absence of Top1 or RNase Hs, R-loops accumulated to substantially higher extent when guanine-runs were located on the non-transcribed strand. This coincides with the orientation where higher genome instability was observed. However, we further report that there are significant differences between the disruption of RNase Hs and Top1 in regards to the orientation-specific elevation in genome instability at the guanine-rich sequence. Additionally, genome instability in Top1-deficient yeasts is not completely suppressed by removal of negative supercoils and further aggravated by expression of mutant Top1. Together, our data provide a strong support for a function of Top1 in suppressing genome instability at the guanine-run containing sequence that goes beyond preventing the transcription-associated RNA:DNA hybrid formation.
authors.affiliation: Department of Microbiology and Molecular Genetics, University of Texas Health Science Center at Houston, Houston, TX 77030, USA.
authors.forename: Puja
authors.initials: P
authors.lastname: Yadav
authors.affiliation: Department of Microbiology and Molecular Genetics, University of Texas Health Science Center at Houston, Houston, TX 77030, USA.
authors.forename: Norah
authors.initials: N
authors.lastname: Owiti
authors.affiliation: Department of Microbiology and Molecular Genetics, University of Texas Health Science Center at Houston, Houston, TX 77030, USA nayun.kim@uth.tmc.edu.
authors.forename: Nayun
authors.initials: N
authors.lastname: Kim
mesh_terms: D002166:Camptothecin; D004261:DNA Replication; D004264:DNA Topoisomerases, Type I / Q000235:genetics / Q000378:metabolism*; D004271:DNA, Fungal / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism; D054856:G-Quadruplexes; D005816:Genetic Complementation Test; D042822:Genomic Instability; D006147:Guanine / Q000737:chemistry / Q000378:metabolism; D007134:Immunoglobulin Switch Region / Q000235:genetics; D009693:Nucleic Acid Hybridization; D012331:RNA, Fungal / Q000235:genetics / Q000378:metabolism*; D011995:Recombination, Genetic; D016914:Ribonuclease H / Q000235:genetics / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000187:drug effects / Q000235:genetics; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic
chemical_list: D004271:DNA, Fungal; D029968:Escherichia coli Proteins; D012331:RNA, Fungal; D029701:Saccharomyces cerevisiae Proteins; D006147:Guanine; D016914:Ribonuclease H; D004264:DNA Topoisomerases, Type I; D002166:Camptothecin