Exploitation of DarT/DarG biology for a rational design of antimicrobial agents

Antibiotic resistant "superbugs" represent a major public health concern of the last 21th century, with an expected number of ten million deaths per year by 2050. Antimicrobial resistance occurs in a wide range of infectious diseases and it is promoted by the activation of pathogen resistance/defence mechanisms, which allow the cells to become persistant and tolerant to antibiotic treatments[1]. Therefore, the design of alternative therapeutic interventions, able to bypass the development of resistance, is urgently needed. Bacterial Toxin-Anti-toxin (TA) systems, widespread in Gram-negative and Gram-positive bacteria, are involved in cell regulatory mechanisms in response to stress stimuli, including antibiotic resistance. Mechanistically, several bacterial endotoxins, belonging to type II TA pairs, induce the downregulation of cell metabolism, targeting endogenous substrates by post-translational modifications. Among these, ADP-ribosylation is emerging as a new player in the field of TA systems. Highly conserved across the evolution[2], ADP-ribosylation is an ancient homeostatic and stress response control system, which relies on the transfer of ADP-ribose unit(s) onto cellular macromolecules. Recently, the DarT/DarG toxin/antitoxin pair was described in Thermus aquaticus, Mycobacterium tuberculosis and Escherichia coli. DarT ADP-ribosylates thymidine in single-stranded DNA in a sequence-consensus manner inducing the impairment of cell growth, while the antitoxin DarG reverses this modification, thereby rescuing cells from DarT toxic effect. Herein we discuss the structural and mechanistic aspects of DarT/DarG toxin-antitoxin-mediated control of DNA ADP-ribosylation, as well as the therapeutic perspectives that the chemical inhibition of this specific ADP-ribosylation signaling may have[3].