Physical and functional interaction between d-ribokinase and topoisomerase I has opposite effects on their respective activity in Mycobacterium smegmatis and Mycobacterium tuberculosis

d-ribose is an essential component of multiple important biological molecules and must first be phosphorylated by ribokinase before entering metabolic pathways. However, the function and regulation of ribokinases in Mycobacterium tuberculosis, the causative agent of tuberculosis, and its related species are largely unknown. In this study, we have characterized the activities of two putative ribokinases, Rv2436 and Ms4585, from M. tuberculosis and Mycobacterium smegmatis, respectively. The mycobacterial topoisomerase I (TopA) was found to physically interact with its ribokinase both in vitro and in vivo. By creating two ribokinase mutants that showed defective interactions with TopA, we further showed that the interaction between ribokinase and TopA had opposite effects on their respective function. While the interaction between the two proteins inhibited the ability of TopA to relax supercoiled DNA, it stimulated ribokinase activity. A cross-regulation assay revealed that the interaction between the two proteins was conserved in the two mycobacterial species. Thus, we uncovered an interplay between ribokinase and topoisomerase I in mycobacteria, which implies the existence of a novel regulatory strategy for efficient utilization of d-ribose in M. tuberculosis that may be useful in stressful environments with restricted access to nutrients.