Identification of Up47 in three thermophilic archaea, one mesophilic archaeon, and one hyperthermophilic bacterium

ArcS from Thermococcus kodakarensis transfers L-lysine to preQ0 nucleoside derivatives as minimum substrate RNAs

Archaeosine (G+) is an archaea-specific tRNA modification synthesized via multiple steps. In the first step, archaeosine tRNA guanine transglucosylase (ArcTGT) exchanges the G15 base in tRNA with 7-cyano-7-deazaguanine (preQ0). In Euryarchaea, preQ015 in tRNA is further modified by archaeosine synthase (ArcS). Thermococcus kodakarensis ArcS catalyzes a lysine-transfer reaction to produce preQ0-lysine (preQ0-Lys) as an intermediate. The resulting preQ0-Lys15 in tRNA is converted to G+15 by a radical S-adenosyl-L-methionine enzyme for archaeosine formation (RaSEA), which forms a complex with ArcS. Here, we focus on the substrate tRNA recognition mechanism of ArcS. Kinetic parameters of ArcS for lysine and tRNA-preQ0 were determined using a purified enzyme. RNA fragments containing preQ0 were prepared from Saccharomyces cerevisiae tRNAPhe-preQ015. ArcS transferred 14C-labeled lysine to RNA fragments. Furthermore, ArcS transferred lysine to preQ0 nucleoside and preQ0 nucleoside 5'-monophosphate. Thus, the L-shaped structure and the sequence of tRNA are not essential for the lysine-transfer reaction by ArcS. However, the presence of D-arm structure accelerates the lysine-transfer reaction. Because ArcTGT from thermophilic archaea recognizes the common D-arm structure, we expected the combination of T. kodakarensis ArcTGT and ArcS and RaSEA complex would result in the formation of preQ0-Lys15 in all tRNAs. This hypothesis was confirmed using 46 T. kodakarensis tRNA transcripts and three Haloferax volcanii tRNA transcripts. In addition, ArcTGT did not exchange the preQ0-Lys15 in tRNA with guanine or preQ0 base, showing that formation of tRNA-preQ0-Lys by ArcS plays a role in preventing the reverse reaction in G+ biosynthesis.

High-resolution crystal structure of RNA kinase ArK1 from G. acetivorans

U47 phosphorylation (Up47) is a novel tRNA modification discovered recently; it can confer thermal stability and nuclease resistance to tRNAs. U47 phosphorylation is catalyzed by Archaeal RNA kinase (Ark1) in an ATP-dependent manner. However, the structural basis for tRNA and/or ATP binding by Ark1 is unclear. Here, we report the expression, purification, and crystallization studies of Ark1 from G. acetivorans (GaArk1). In addition to the Apo-form structure, one GaArk1-ATP complex was also determined in atomic resolution and revealed the detailed basis for ATP binding by GaArk1. The GaArk1-ATP complex represents the only ATP-bound structure of the Ark1 protein. The majority of the ATP-binding residues are conserved, suggesting that GaArk1 and the homologous proteins share similar mechanism in ATP binding. Sequence and structural analysis further indicated that endogenous guanosine will only inhibit the activities of certain Ark1 proteins, such as Ark1 from T. kodakarensis.

Transfer RNA modifications and cellular thermotolerance

RNA molecules are modified post-transcriptionally to acquire their diverse functions. Transfer RNA (tRNA) has the widest variety and largest numbers of RNA modifications. tRNA modifications are pivotal for decoding the genetic code and stabilizing the tertiary structure of tRNA molecules. Alternation of tRNA modifications directly modulates the structure and function of tRNAs and regulates gene expression. Notably, thermophilic organisms exhibit characteristic tRNA modifications that are dynamically regulated in response to varying growth temperatures, thereby bolstering fitness in extreme environments. Here, we review the history and latest findings regarding the functions and biogenesis of several tRNA modifications that contribute to the cellular thermotolerance of thermophiles.

Fluorescent labeling of tRNA for rapid kinetic interaction studies with tRNA-binding proteins

Transfer RNA (tRNA) plays a critical role during translation and interacts with numerous proteins during its biogenesis, functional cycle and degradation. In particular, tRNA is extensively post-transcriptionally modified by various tRNA modifying enzymes which each target a specific nucleotide at different positions within tRNAs to introduce different chemical modifications. Fluorescent assays can be used to study the interaction between a protein and tRNA. Moreover, rapid mixing fluorescence stopped-flow assays provide insights into the kinetics of the tRNA-protein interaction in order to elucidate the tRNA binding mechanism for the given protein. A prerequisite for these studies is a fluorescently labeled molecule, such as fluorescent tRNA, wherein a change in fluorescence occurs upon protein binding. In this chapter, we discuss the utilization of tRNA modifications in order to introduce fluorophores at particular positions within tRNAs. Particularly, we focus on in vitro thiolation of a uridine at position 8 within tRNAs using the tRNA modification enzyme ThiI, followed by labeling of the thiol group with fluorescein. As such, this fluorescently labeled tRNA is primarily unmodified, with the exception of the thiolation modification to which the fluorophore is attached, and can be used as a substrate to study the binding of different tRNA-interacting factors. Herein, we discuss the example of studying the tRNA binding mechanism of the tRNA modifying enzymes TrmB and DusA using internally fluorescein-labeled tRNA.