PSF/p54(nrb) stimulates "jumping" of DNA topoisomerase I between separate DNA helices

We have previously shown [Straub et al. (1998) J. Biol. Chen. 273, 26261] t hat the pyrimidine tract binding protein associated splicing factor PSF/p54 (nrb) binds and stimulates DNA topoisomerase I. Here we show that cleavage and religation half-reactions of topoisomerase I are unaffected by PSF/p54( nrb), whereas the propensity of the enzyme to jump between separate DNA hel ices is stimulated. To demonstrate such an effect, topoisomerase I was firs t captured in suicidal cleavage of an oligonucleotide substrate. Subsequent ly, a cleavage/ligation equilibrium was established by adding a ligation do nor under conditions allowing recleavage of the ligated substrate. Finally, a second oligonucleotide was added to the mixture, which also allowed suic idal cleavage by topoisomerase I, but did not accommodate the ligation dono r of the first oligonucleotide. Thus, topoisomerase I was given the choice to engage in repeated cleavage/ligation cycles of the first oligonucleotide or to jump to the second suicide substrate and get trapped. PSF/p54(nrb) e nhanced the cleavage rate of the second oligonucleotide (11-fold), suggesti ng that it stimulates the dissociation of topoisomerase I after ligation. T hus, stimulation of topoisomerase I catalysis by PSF/p54(nrb) seems to be a ffected by mobilization of the enzyme.