Cloning of cDNA encoding a novel mouse DNA topoisomerase III (Topo IIIβ) possessing negatively supercoiled DNA relaxing activity, whose message is highly expressed in the testis

We cloned cDNA encoding a novel mouse homologue of DNA topoisomerase III (mTOP3 beta). The nucleotide sequence contains an open reading frame of 863 amino acids, and the deduced molecular mass of the coded protein is 96.9 kDa. The overall sequence of mTOP3 beta has a 48 and 36% identity with mouse TOP3 alpha at the nucleotide and amino acid level, respectively. DNA topoisomerase III beta was expressed using a baculovirus expression system and purified. The purified DNA topoisomerase mp had activity to relax negatively supercoiled DNA. Relaxation of supercoiled DNA was partial at 37 degrees C and complete relaxation was observed at higher temperatures. mTOP3 beta mRNA was strongly expressed in the testis and relatively strongly in the brain. The levels of TOP3 beta mRNA in the testis increased slightly 14 days and considerably 17 days after birth, when the cells in the pachytene phase bean to appear and increase.