Molecular mechanics and DNA replication regulation.

ABSTRACT

Magnetic and optical tweezers are providing novel insights on the structure, energetics, and functional dynamics of biological macromolecules. The modulation of DNA topology has provided very appropriate opportunities to study with these technologies the energetic and mechanistic features of the action of DNA topoisomerases, the enzymes that maintain the physiological level of negative superhelicity in the genome. Modulation of the superhelical state of the DNA replication origins is essential for the initiation of DNA synthesis in prokaryotes and eukaryotes. The results obtained recently from independent studies of two different groups combine to give new insights on the topological aspects of this process. With magnetic tweezers it was shown that the inhibition of human topoisomerase I by camptothecin freezes the drug-enzyme-DNA complex and specifically forbids the relaxation of positive supercoils; a study of the in vivo role of topoisomerase I on the activation of a human origin showed that this process is forbidden when the enzyme is frozen on the origin DNA by camptothecin. The inhibition of the relaxation of positive supercoils, probably introduced by the proteins performing origin activation, is therefore lethal for this process. Thus, the use of advanced physical technologies provides insights on an essential biological process.