ADP-Ribosylation of Cytidine: A Novel Nucleic Acid Modification Reversed by NADAR Hydrolases

ADP-ribosylation of nucleic acids is a modification found in both eukaryotes and bacteria, where it contributes to genome maintenance but can also serve as a toxic mechanism used by bacterial toxins to disrupt essential cellular processes. This modification is catalysed by ADP-ribosyltransferases and can be reversed by antagonistic ADP-ribosylgylcohydrolase enzymes. To date, ADP-ribosylation of nucleic acid bases has been described only for adenosine, guanosine, and thymidine. Here we report the ADP-ribosylation of cytidine, catalysed by members of the pierisin family of bacterial toxins-ScARP (SCO5461) and Scabin. We also show that ADP-ribosylation of cytidine is reversible through removal by certain NADAR family proteins, including NADAR proteins from the bacterium <i>Streptomyces coelicolor</i> (SCO5665) and the sponge <i>Amphimedon queenslandica</i>, as well as YbiA-type NADAR proteins. The conservation of cytidine de-ADP-ribosylating activity of NADAR proteins across phylogenetically distant species suggests that this modification may have important physiological significance.