NAD in RNA: unconventional headgear.

Although widely assumed to bear a 5'-terminal triphosphate or monophosphate, recent evidence suggests that the 5' end of bacterial RNA can sometimes bear a modification reminiscent of a eukaryotic cap. A new study has now identified Escherichia coli RNAs that begin with a noncanonical cap resembling the redox cofactor nicotinamide adenine dinucleotide (NAD), as well as a cellular enzyme that can remove it. The biological function of such caps remains to be determined.