Theoretical basis for stabilizing messenger RNA through secondary structure design

RNA hydrolysis presents problems in manufacturing, long-term storage, world-wide delivery, and in vivo stability of messenger RNA (mRNA)-based vaccines and therapeutics. A largely unexplored strategy to reduce mRNA hydrolysis is to redesign RNAs to form double-stranded regions, which are protected from in-line cleavage and enzymatic degradation, while coding for the same proteins. The amount of stabilization that this strategy can deliver and the most effective algorithmic approach to achieve stabilization remain poorly understood. Motivated by the need for stabilized COVID-19 mRNA vaccines, we present simple calculations for estimating RNA stability against hydrolysis, and a model that links the average unpaired probability of an mRNA, or AUP, to its overall rate of hydrolysis. To characterize the stabilization achievable through structure design, we compare optimization of AUP by conventional mRNA design methods to results from the LinearDesign algorithm, a new Monte Carlo tree search algorithm called RiboTree, and crowdsourcing through the OpenVaccine challenge on the Eterna platform. Tests were carried out on mRNAs encoding nanoluciferase, green fluorescent protein, and COVID-19 mRNA vaccine candidates encoding SARS-CoV-2 epitopes, spike receptor binding domain, and full-length spike protein. We find that Eterna and RiboTree significantly lower AUP while maintaining a large diversity of sequence and structure features that correlate with translation, biophysical size, and immunogenicity. Our results suggest that increases in in vitro mRNA half-life by at least two-fold are immediately achievable and that further stability improvements may be enabled with thorough experimental characterization of RNA hydrolysis.

A multifunctional type IA DNA/RNA topoisomerase with RNA hydrolysis and rRNA processing activities from Mycobacterium smegmatis and Mycobacterium tuberculosis

Topoisomerases maintain topological homeostasis of bacterial chromosomes by catalysing changes in DNA linking number. The resolution of RNA entanglements occurring in the cell would also require catalytic action of topoisomerases. We describe RNA topoisomerase and hydrolysis activities in DNA topoisomerase I (topo I) from mycobacteria. The interaction of topo I with mRNA, tRNA and rRNA suggested its role in some aspect of RNA metabolism; the enzyme participates in rRNA maturation via its RNA hydrolysis activity. Accumulation of rRNA precursors in a topo I knockdown strain and the rescue of rRNA processing deficiency in RNaseE knockdown cells by topo I expression, indicated the enzyme’s back-up support to RNases involved in rRNA processing. We demonstrate that the active site tyrosine of the enzyme mediates catalytic reactions with both DNA/RNA substrates, and RNA topoisomerase activity can follow two reaction paths in contrast to its DNA topoisomerase activity. Mutation in the canonical proton relay pathway impacts DNA topoisomerase activity while retaining activity on RNA substrates. The mycobacterial topo I thus exemplifies the resourcefulness and parsimony of biological catalysis in harnessing the limited chemical repertoire at its disposal to find common solutions to mechanistically-related challenges of phosphodiester breakage/exchange reactions in DNA and RNA that are essential for cell survival.

High intrinsic hydrolytic activity of cyanobacterial RNA polymerase compensates for the absence of transcription proofreading factors

The vast majority of organisms possess transcription elongation factors, the functionally similar bacterial Gre and eukaryotic/archaeal TFIIS/TFS. Their main cellular functions are to proofread errors of transcription and to restart elongation via stimulation of RNA hydrolysis by the active centre of RNA polymerase (RNAP). However, a number of taxons lack these factors, including one of the largest and most ubiquitous groups of bacteria, cyanobacteria. Using cyanobacterial RNAP as a model, we investigated alternative mechanisms for maintaining a high fidelity of transcription and for RNAP arrest prevention. We found that this RNAP has very high intrinsic proofreading activity, resulting in nearly as low a level of in vivo mistakes in RNA as Escherichia coli. Features of the cyanobacterial RNAP hydrolysis are reminiscent of the Gre-assisted reaction—the energetic barrier is similarly low, and the reaction involves water activation by a general base. This RNAP is resistant to ubiquitous and most regulatory pausing signals, decreasing the probability to go off-pathway and thus fall into arrest. We suggest that cyanobacterial RNAP has a specific Trigger Loop domain conformation, and isomerises easier into a hydrolytically proficient state, possibly aided by the RNA 3′-end. Cyanobacteria likely passed these features of transcription to their evolutionary descendants, chloroplasts.