Single-strand deoxyribonucleic acid binding protein from rat liver changes the helical structure of deoxyribonucleic acid.

Incubation of rat liver single-strand DNA binding protein S25 with covalently closed relaxed SV40 DNA in the presence of rat liver topoisomerase I induced a decrease in the linking number LK of DNA, so that it appeared negatively supertwisted after removal of the protein. delta LK was found to be a linear function of protein to DNA ratio and reached a plateau corresponding to about 15 superhelical turns. The same result was obtained when S25 was incubated with form I or form Ir before addition of topoisomerase I or when SV40 was replaced by PM2 DNA. The observed reduction in the linking number of DNA when it is closed in the presence of rat liver protein S25 can be explained either by supercoiling of DNA induced by S25 or by detorsion or unwinding of DNA.