DNA RELAXATION MEDIATED BY USTILAGO-MAYDIS TYPE-I TOPOISOMERASE - MODULATION BY CHROMATIN ASSOCIATED PROTEINS

Ustilago maydis topoisomerase I relaxes superhelical DNA in the absenc e of any co-factors. The reaction reaches a defined end-point proporti onal to the amount of enzyme added and an analysis of the reaction by Hill plot transformation indicates that at least two molecules of topo isomerase must interact with the DNA to catalyze relaxation. The addit ion of purified Ustilago histone H1 reduces the stoichiometric amount of topoisomerase I required by 50%. H1 histone may function to enhance DNA relaxation through a cooperative mechanism. The purified HMG-like protein from Ustilago also enhances DNA relaxation mediated by the to poisomerase. Whereas H1 stimulates topo I-mediated DNA relaxation thro ugh a processive mode, the HMG-like protein enhances through a distrib utive mechanism. Taken together, these results demonstrate that the in teraction of chromosomal proteins with topoisomerase can influence DNA topology, and mechanisms are proposed to explain this enhancement.