NAD+ inhibits the self-splicing of the group I intron

Abstract

We investigated the effects of the coenzyme NAD+ (nicotinamide adenine dinucleotide) and its analogs on the self-splicing of primary transcripts of the phage T4 thymidylate synthase gene (td). Of all the nicotinamide coenzymes and analogs tested, NADP+ was the strongest inhibitor, with a potency approximately threefold that of NAD+. Kinetic analysis demonstrated that NAD+ acts as a mixed type noncompetitive inhibitor for the td intron RNA with a K(i) of 4.1 mM. The splicing specificity inhibition by NAD+ is predominantly due to changes in Km and kcat, and was Mg2+ concentration dependent. The results suggest that both the ADP and nicotinamide moieties are the key structural features in NAD+ responsible for the inhibition of splicing.