ts A1S9 locus in mouse L cells may encode a novobiocin binding protein that is required for DNA topoisomerase II activity.

Abstract

Nuclear novobiocin binding proteins (NBPs) from a set of mouse L cells have been extensively purified by affinity chromatography on novobiocin-Sepharose columns. The NBPs, specifically eluted with 100 ,ug of novobiocin per ml, exhibited equivalent DNA topoisomerase activities (measured as ATP-dependent relaxation or catenation of #X174 replicative-form I DNA substrate) when extracted from equal numbers of wild-type (WT-4) mouse L cells growing logarithmically at 34WC or at 38.50C, from ts A1S9 cells similarly cultivated at the low, permissive temperature or from revertant ts+ AR cells in exponential growth at either temperature. The NBPs isolated from similar numbers of ts A1S9 cells grown to midlogarithmic phase and then incubated for 24 hr at 38.5C (the nonpermissive temperature) showed no topoisomerase II activity. Preliminary NaDodSO4/polyaerylamide gel electrophoretic analysis of enzymatically active material revealed that the NBPs of WT4 and ts+ AR cells grown at 34WC comprised three major polypeptides of 76,000, 74,000, and 30,000 daltons and a number of larger molecular mass components present in trace amounts. The NBP of ts A1S9 cells grown at the permissive temperature was similar, except that the 30,000-kilodalton polypeptide was not detected. Such enzymatically active NBPs from WT-4 and ts+ AR cells were unaffected by 100 1tg of novobiocin per ml, whereas the analogous preparation from ts A1S9 cells was totally inhibited. On the basis of these and other considerations, it is postulated that the ts A1S9 locus of mouse L cells encodes a temperature-sensitive polypeptide that is required for normal DNA topoisomerase I activity.The ts AlS9 L cell (1) is mutant in a gene required for nuclear DNA replication (2, 3) and for normal progression through the S phase of the cell cycle (4). Temperature-inactivated ts A1M9 cells synthesize "Okazald fragments" and convert them to progeny single-strand DNA of >5 x 106 daltons (2). In addition they support full replication of monomeric and multimeric (up to 34 X 107 daltons) polyoma DNA (5, 6). Therefore, it was concluded that the enzyme proteins of polydeoxyribonucleotide guished by their properties and by the mechanism by which they modify the topology of DNA. The DNA topoisomerase I more closely resembles the nicking-closing, w protein activity already well characterized biochemically and genetically in bacteria (12, 13). Whereas the topoisomerase I enzyme has long been recognized to function in eukaryotes (14), evidence for the topoisomerase II eukaryotic analogue for the DNA gyrase of prokaryotes was reported later (15-17) and, indeed, after the studies described herein were initiated. No evidence for abnormal topoisomerase I activity had been seen in earlier studies of temperature-inactivated ts AlS9 cells (cf. refs. 4 and 6; unpublished data). Therefore, attention was focused on DNA topoisomerase II. The first hint that this enzyme might be affected came with the demonstration that ts AlS9 cells are hypersensitive to novobiocin (9) The publication co...

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