Roles of DNA topoisomerase II in the development of the cellular slime molds.

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To study the relationship between gene expression and DNA topology, the DNA topoisomerase II gene of the cellular slime mold Dictyostelium discoideum was cloned and sequenced.Totally, 6.3 Kb region containing a DNA topoisomerase II gene was cloned using several oligonucleotide probes which have conserved amino acid sequence regions among the genes of several organisms. The 5.5 Kb region sequenced contained a open reading frame of 3,849 nucleotides which correspond to 1,282 amino acids and to 146 KDa protein. Homology search showed that the ORF has very similar amino acid sequence to DNA topoisomerase II of eucaryote and DNA gyrase a plus DNA gyrase b. Using Southern blot experiment, the gene was shown to be a single copy in the genome of D. discoideum. Northern blot analysis revealed that the transcript of the gene is about 4,100 nucleotides long and the amount of the transcript is low because the transcript could be detected only using poly(A)^+ RNA fraction. To make antibody against the DNA topoisomerase II, the DNA region spanning 1.9 Kb of the gene was inserted in the downstream of glutathion-S-transferase gene of the E. coli expression vector pGEX. The denatured, fusion protein was purified from inclusion body of E. coli and injected into a rabbit. Antibody was recovered from immune serum of the rabbit. Western blot using the antibody showed that the antibody recognized a 140 KDa protein of whole homogenate of D. discoideum cell. However, the antibody did not cross-react with any proteins of D. mucoroides cell and cross-reacted with about 220 KDa protein of Polyshondylium pallidum cell. The amount of the enzyme protein gradually decreased during from exponentially growing phase to stationary phase and during development.