Abundance of nuclear DNA topoisomerase II is correlated with proliferation in Arabidopsis thaliana.

Topoisomerase II (TOPII) is an important enzyme involved in DNA replication and chromosome condensation. The level of TOPII expression has been correlated with the proliferative state of eukaryotic cells. Here we report the cloning and characterization of a cDNA clone AtTopII encoding the first reported TOPII from higher plants. AtTopII is 4603 base pairs (bp) in length and encodes an open reading frame of 1473 amino acid residues. One interesting feature of AtTopII is the presence of a 110 bp direct repeat in the last one-third of the cDNA. Analysis of the genomic sequence within this region by PCR revealed that this duplication includes a small intron of 89 bp. Conservation of sequences within this repeated intron suggests that this in-frame duplication may be a relatively recent event. The deduced amino acid sequence of AtTopII shows strong homologies to TOPII sequences reported from other eukaryotes, particularly in the regions that are highly conserved among different species. Southern blot analysis with Arabidopsis DNA indicates that AtTopII is a single-copy gene while Northern blots detected a 5.0 kb transcript, the level of which is substantially higher in young seedlings than in mature plants. Using a polyclonal antiserum raised against the C-terminal one-third of AtTOPII, we found that the protein is localized in the nucleus and its level is correlated with the proliferative state of the particular tissue.