Reverse gyrase is the only topoisomerase that can introduce positive supercoils into DNA in an ATP-dependent process. It has a modular structure and harnesses a helicase-like domain to support a topoisomerase activity, thereby creating the unique function of positive DNA supercoiling. The isolated topoisomerase domain can relax negatively supercoiled DNA, an activity that is suppressed in reverse gyrase. The isolated helicase-like domain is a nucleotide-dependent switch that is attenuated by the topoisomerase domain. Inter-domain communication thus appears central for the functional cooperation of the two domains. The latch, an insertion into the helicase-like domain, has been suggested as an important element in coordinating their activities. Here, we have dissected the influence of the latch on nucleotide and DNA binding to the helicase-like domain, and on DNA supercoiling by reverse gyrase. We find that the latch is required for positive DNA supercoiling. It is crucial for the cooperativity of DNA and nucleotide binding to the helicase-like domain. The latch contributes to DNA binding, and affects the preference of reverse gyrase for ssDNA. Thus, the latch coordinates the individual domain activities by modulating the helicase-like domain, and by communicating changes in the nucleotide state to the topoisomerase domain.