Hmo1, an HMG-box protein, belongs to the yeast ribosomal DNA transcription system

Hmo1 is one of seven HMG-box proteins of Saccharo myces cerevisiae. Null mutants have a limited effect on growth. Hmo1 overexpression suppresses rpa49-Δ mutants lacking Rpa49, a non-essential but conserved subunit of RNA polymerase I corresponding to the animal RNA polymerase I factor PAF53. This overexpression strongly increases de novo rRNA synthesis. rpa49-Δ hmo1-Δ double mutants are lethal, and this lethality is bypassed when RNA polymerase II synthesizes rRNA. Hmo1 co-localizes with Fob1, a known rDNA-binding protein, defining a narrow territory adjacent to the nucleoplasm that could delineate the rDNA nucleolar domain. These data identify Hmo1 as a genuine RNA polymerase I factor acting synergistically with Rpa49. As an HMG-box protein, Hmo1 is remotely related to animal UBF factors. hmo1-Δ and rpa49-Δ are lethal with top3-Δ DNA topoisomerase (type I) mutants and are suppressed in mutants lacking the Sgs1 DNA helicase. They are not affected by top1-Δ defective in Top1, the other eukaryotic type I topoisomerase. Conversely, rpa34-Δ mutants lacking Rpa34, a non-essential subunit associated with Rpa49, are lethal in top1-Δ but not in top3-Δ.