Portal de Periódicos da CAPES

The mechanism of RNA 5′ capping with NAD+, NADH and desphospho-CoA

2016; Nature Portfolio; Volume: 535; Issue: 7612 Linguagem: Inglês

10.1038/nature18622

1476-4687

Jeremy G. Bird, Yu Zhang, Yuan Tian, Natalya Panova, Ivan Barvı́k, L. I. Greene, Min Liu, Brian Buckley, Libor Krásný, Jeehiun K. Lee, Craig D. Kaplan, Richard H. Ebright, Bryce E. Nickels,

RNA Research and Splicing

RNA caps other than the 7-methylguanylate modification are generated by a distinct mechanism in which caps are added during, not after, transcription initiation through the use of non-canonical initiating nucleotides by RNA polymerases, a finding which has functional consequences. To prevent degradation, both ends of messenger RNAs are modified to block nucleases. Until recently, the only modification known to protect the 5' end was a 7-methylguanylate 'cap', although it has been found that some mRNAs use NAD+ or dpCoA instead. It has been assumed that the mechanism of addition of these alternative caps would be the same. However, Bryce Nickels and colleagues now show that alternative caps use a distinct mechanism, by which they are incorporated during, not after, transcription initiation. In both bacterial and eukaryotic cells, RNA polymerase uses these moieties as non-canonical initiating nucleotides, and their use has functional consequences. The chemical nature of the 5′ end of RNA is a key determinant of RNA stability, processing, localization and translation efficiency1,2, and has been proposed to provide a layer of 'epitranscriptomic' gene regulation3. Recently it has been shown that some bacterial RNA species carry a 5′-end structure reminiscent of the 5′ 7-methylguanylate 'cap' in eukaryotic RNA. In particular, RNA species containing a 5′-end nicotinamide adenine dinucleotide (NAD+) or 3′-desphospho-coenzyme A (dpCoA) have been identified in both Gram-negative and Gram-positive bacteria3,4,5,6. It has been proposed that NAD+, reduced NAD+ (NADH) and dpCoA caps are added to RNA after transcription initiation, in a manner analogous to the addition of 7-methylguanylate caps6,7,8. Here we show instead that NAD+, NADH and dpCoA are incorporated into RNA during transcription initiation, by serving as non-canonical initiating nucleotides (NCINs) for de novo transcription initiation by cellular RNA polymerase (RNAP). We further show that both bacterial RNAP and eukaryotic RNAP II incorporate NCIN caps, that promoter DNA sequences at and upstream of the transcription start site determine the efficiency of NCIN capping, that NCIN capping occurs in vivo, and that NCIN capping has functional consequences. We report crystal structures of transcription initiation complexes containing NCIN-capped RNA products. Our results define the mechanism and structural basis of NCIN capping, and suggest that NCIN-mediated 'ab initio capping' may occur in all organisms.