NAD+ 5’ capped RNA have been found in yeast, humans, and Arabidopsis thaliana. In eukaryotes, the NAD+ cap is removed by non-canonical decapping enzymes from the DXO family. DeNADing by DXO results in a 5’ end monophosphate RNA distinct from NudC which results in NMN plus 5′ monophosphate RNA. Importantly, DXO is ~6 fold more efficient at decapping NAD+ compared to m7G, suggesting that it selectively degrades NAD-capped RNA rather than the more common m7G cap, similar to NudC.
The m7G cap has been shown to promote translation through recruitment of the initiation complex onto the mRNA. However, the NAD+-cap does not provide a similar function as NAD+-capped and polyadenylated mRNA displayed similar levels of translation in vitro to uncapped mRNA. Additionally, the 5’ NAD+ cap further promotes decay of the RNA it is attached to, NAD+-capped and polyadenylated mRNA were demonstrated in vitro to be less stable than mRNAs lacking a 5’ cap, suggesting that the NAD+ modification is actively facilitating DXO-mediated RNA decay.
While the relationship between RNA-binding proteins, such as glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and NAD+ concentration is established, the NAD+ cap has been hypothesized to represent a direct link between RNA expression levels and cellular metabolism. It is known that energy stresses such as glucose deprivation and caloric restriction influence NAD+ concentrations and can possibly impact NAD+ capping. Additionally, as low-nutrient conditions can affect mRNA stability, and seeing as NAD+ caps promote mRNA decay, it is possible that the energetic state of a cell could affect NAD+-capping and thus mRNA turnover. Certain findings, such as the higher abundance of NAD+-capped transcripts in stationary-phase bacteria as well as yeast grown on synthetic media, point toward this possibility.