Deciphering the RNA capping process in bacteria

Jonathan Jagodnik and Richard L. Gourse

PNAS March 3, 2020 117 (9) 4445-4446

RNA capping in eukaryotes has been studied since the 1970s, starting with the discovery of 5′ 7-methylguanylate caps in the Shatkin laboratory (1). That capping mechanism involves a pause during transcription elongation that allows the recruitment of specialized capping enzymes to modify the 5′ end of the nascent RNA. In bacteria, however, RNA capping was observed only within the past decade when derivatives of either coenzyme A (CoA) (2) or nicotinamide adenine dinucleotide (NAD) (3, 4) were reported at some RNA 5′ ends. The first bacterial capping mechanism was established convincingly in 2016 when it was observed that RNA polymerase (RNAP) could incorporate caps into RNA by using NAD and CoA as noncanonical initiating nucleotides (5). Last year, the Belasco laboratory at New York University demonstrated that a different type of cap, namely nucleoside tetraphosphate (Np₄), is found at the 5′ end of RNAs in Escherichia coli (6) under conditions where dinucleoside tetraphosphates (Np₄N) are increased. In PNAS, Luciano and Belasco (7) demonstrate the ability of E. coli RNAP to use Np₄N as the initiating nucleotide and its dependence on promoter sequence. In some cases, incorporation of an Np₄N is strongly favored over initiation with standard NTPs. They thereby establish that RNAP is the main Np₄ messenger RNA (mRNA) capping machinery used in E. coli and that the cap is incorporated during transcription initiation, in contrast with the postinitiation addition mechanism employed in eukaryotes.

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In summary, Np₄ capping provides an underappreciated mechanism for regulation of gene expression in bacteria. As its role is explored further, it seems likely that it will be found to play a role in responses to environmental and nutritional changes that have previously been unexplained.

See more: https://www.pnas.org/content/117/9/4445