NAD+-capped RNAs are widespread in rice (Oryza sativa) and spatiotemporally modulated during development

Bridging the gap: RNAylation conjugates RNAs to proteins

In nature, the majority of known RNA-protein interactions are transient. Our recent study has depicted a novel mechanism known as RNAylation, which covalently links proteins and RNAs. This novel modification bridges the realms of RNA and protein modifications. This review specifically explores RNAylation catalyzed by bacteriophage T4 ADP-ribosyltransferase ModB, with a focus on its protein targets and RNA substrates in the context of Escherichia coli-bacteriophage T4 interaction. Additionally, we discuss the biological significance of RNAylation and present perspectives on RNAylation as a versatile bioconjugation strategy for RNAs and proteins.

Toll/interleukin-1 receptor (TIR) domain-containing proteins have NAD-RNA decapping activity

The occurrence of NAD+ as a non-canonical RNA cap has been demonstrated in diverse organisms. TIR domain-containing proteins present in all kingdoms of life act in defense responses and can have NADase activity that hydrolyzes NAD+. Here, we show that TIR domain-containing proteins from several bacterial and one archaeal species can remove the NAM moiety from NAD-capped RNAs (NAD-RNAs). We demonstrate that the deNAMing activity of AbTir (from Acinetobacter baumannii) on NAD-RNA specifically produces a cyclic ADPR-RNA, which can be further decapped in vitro by known decapping enzymes. Heterologous expression of the wild-type but not a catalytic mutant AbTir in E. coli suppressed cell propagation and reduced the levels of NAD-RNAs from a subset of genes before cellular NAD+ levels are impacted. Collectively, the in vitro and in vivo analyses demonstrate that TIR domain-containing proteins can function as a deNAMing enzyme of NAD-RNAs, raising the possibility of TIR domain proteins acting in gene expression regulation.

A viral ADP-ribosyltransferase attaches RNA chains to host proteins

The mechanisms by which viruses hijack the genetic machinery of the cells they infect are of current interest. When bacteriophage T4 infects Escherichia coli, it uses three different adenosine diphosphate (ADP)-ribosyltransferases (ARTs) to reprogram the transcriptional and translational apparatus of the host by ADP-ribosylation using nicotinamide adenine dinucleotide (NAD) as a substrate1,2. NAD has previously been identified as a 5' modification of cellular RNAs3-5. Here we report that the T4 ART ModB accepts not only NAD but also NAD-capped RNA (NAD-RNA) as a substrate and attaches entire RNA chains to acceptor proteins in an 'RNAylation' reaction. ModB specifically RNAylates the ribosomal proteins rS1 and rL2 at defined Arg residues, and selected E. coli and T4 phage RNAs are linked to rS1 in vivo. T4 phages that express an inactive mutant of ModB have a decreased burst size and slowed lysis of E. coli. Our findings reveal a distinct biological role for NAD-RNA, namely the activation of the RNA for enzymatic transfer to proteins. The attachment of specific RNAs to ribosomal proteins might provide a strategy for the phage to modulate the host's translation machinery. This work reveals a direct connection between RNA modification and post-translational protein modification. ARTs have important roles far beyond viral infections6, so RNAylation may have far-reaching implications.

NAD-capped RNAs - a redox cofactor meets RNA

RNA modifications immensely expand the diversity of the transcriptome, thereby influencing the function, localization, and stability of RNA. One prominent example of an RNA modification is the eukaryotic cap located at the 5' terminus of mRNAs. Interestingly, the redox cofactor NAD can be incorporated into RNA by RNA polymerase in vitro. The existence of NAD-modified RNAs in vivo was confirmed using liquid chromatography and mass spectrometry (LC-MS). In the past few years novel technologies and methods have characterized NAD as a cap-like RNA structure and enabled the investigation of NAD-capped RNAs (NAD-RNAs) in a physiological context. We highlight the identification of NAD-RNAs as well as the regulation and functions of this epitranscriptomic mark in all domains of life.