Molecular cloning and expression of the gene encoding the kinetoplast-associated type II DNA topoisomerase of Crithidia fasciculata.

A type II DNA topoisomerase, topoIImt, was shown previously to be associated with the kinetoplast DNA of the trypanosomatid Crithidia fasciculata. The gene encoding this kinetoplast-associated topoisomerase has been cloned by immunological screening of a Crithidia genomic expression library with monoclonal antibodies raised against the purified enzyme. The gene CfaTOP2 is a single copy gene and is expressed as a 4.8-kb polyadenylated transcript. The nucleotide sequence of CfaTOP2 has been determined and encodes a predicted polypeptide of 1239 amino acids with a molecular mass of 138,445. The identification of the cloned gene is supported by immunoblot analysis of the beta-galactosidase-CfaTOP2 fusion protein expressed in Escherichia coli and by analysis of tryptic peptide sequences derived from purified topoIImt. CfaTOP2 shares significant homology with nuclear type II DNA topoisomerases of other eukaryotes suggesting that in Crithidia both nuclear and mitochondrial forms of topoisomerase II are encoded by the same gene.