RHL1 is an essential component of the plant DNA topoisomerase VI complex and is required for ploidy-dependent cell growth

Proc Natl Acad Sci U S A. 2005 Dec 20;102(51):18736-41. doi: 10.1073/pnas.0505883102. Epub 2005 Dec 8.


How cells achieve their final sizes is a pervasive biological question. One strategy to increase cell size is for the cell to amplify its chromosomal DNA content through endoreduplication cycles. Although endoreduplication is widespread in eukaryotes, we know very little about its molecular mechanisms. Successful progression of the endoreduplication cycle in Arabidopsis requires a plant homologue of archaeal DNA topoisomerase (topo) VI. To further understand how DNA is endoreduplicated and how this process is regulated, we isolated a dwarf Arabidopsis mutant, hyp7 (hypocotyl 7), in which various large cell types that in the wild type normally endoreduplicate multiple times complete only the first two rounds of endoreduplication and stall at 8C. HYP7 encodes the RHL1 (ROOT HAIRLESS 1) protein, and sequence analysis reveals that RHL1 has similarity to the C-terminal domain of mammalian DNA topo IIalpha, another type II topo that shares little sequence homology with topo VI. RHL1 shows DNA binding activity in vitro, and we present both genetic and in vivo evidence that RHL1 forms a multiprotein complex with plant topo VI. We propose that RHL1 plays an essential role in the topo VI complex to modulate its function and that the two distantly related topos, topo II and topo VI, have evolved a common domain that extends their function. Our data suggest that plant topo II and topo VI play distinct but overlapping roles during the mitotic cell cycle and endoreduplication cycle.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Arabidopsis / cytology*
  • Arabidopsis / genetics
  • Arabidopsis / growth & development
  • Arabidopsis / metabolism*
  • Arabidopsis Proteins / chemistry
  • Arabidopsis Proteins / genetics
  • Arabidopsis Proteins / metabolism*
  • Archaeal Proteins
  • Base Sequence
  • Cell Cycle
  • Cell Size
  • Cloning, Molecular
  • DNA / metabolism
  • DNA Topoisomerases, Type II / chemistry*
  • DNA Topoisomerases, Type II / metabolism*
  • DNA-Binding Proteins / chemistry
  • DNA-Binding Proteins / genetics
  • DNA-Binding Proteins / metabolism
  • Molecular Sequence Data
  • Nuclear Proteins / chemistry
  • Nuclear Proteins / genetics
  • Nuclear Proteins / metabolism*
  • Phenotype
  • Ploidies*
  • Protein Binding
  • Sequence Alignment
  • Sequence Homology, Amino Acid


  • Arabidopsis Proteins
  • Archaeal Proteins
  • DNA-Binding Proteins
  • Nuclear Proteins
  • RHL1 protein, Arabidopsis
  • RHL2 protein, Arabidopsis
  • DNA
  • DNA topoisomerase VI
  • DNA Topoisomerases, Type II