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

Abstract

We cloned cDNA encoding a novel mouse homologue of DNA topoisomerase III (mTOP3β). 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 mTOP3β has a 48 and 36% identity with mouse TOP3α at the nucleotide and amino acid level, respectively. DNA topoisomerase IIIβ was expressed using a baculovirus expression system and purified. The purified DNA topoisomerase IIIβ had activity to relax negatively supercoiled DNA. Relaxation of supercoiled DNA was partial at 37 °C and complete relaxation was observed at higher temperatures. mTOP3β mRNA was strongly expressed in the testis and relatively strongly in the brain. The levels of TOP3β mRNA in the testis increased slightly 14 days and considerably 17 days after birth, when the cells in the pachytene phase begin to appear and increase.