Transition protein 4 from boar late spermatid nuclei is a topological factor that stimulates DNA-relaxing activity of topoisomerase I

Transition protein 4 (TP4) from boar late spermatid nuclei, having higher a ffinity for double-stranded DNA and a local melting activity of DNA, stimul ated SV40 DNA-relaxing activity of eukaryotic topoisomerase I at TP4/DNA mo lar ratios of 6,6-11, A TP4-spermidine mixture stimulated the activity of t opoisomerase I much more than spermidine alone, but no more than TP4 alone, and poly-L-arginine did not. These results suggest that TP4 contributes to the chromatin reorganization in the late spermatid nuclei from nucleosomal -type structure with negatively supercoiled DNA to nucleoprotamine structur e with no supercoiled DNA, (C) 1999 Federation of European Biochemical Soci eties.