Pseudouridine synthases catalyze formation of the most abundant modification of functional RNAs by site-specifically isomerizing uridines to pseudouridines. While the structure and substrate specificity of these enzymes have been studied in detail, the kinetic and the catalytic mechanism of pseudouridine synthases remain unknown. Here, the first pre-steady-state kinetic analysis of three Escherichia coli pseudouridine synthases is presented. A novel stopped-flow absorbance assay revealed that substrate tRNA binding by TruB takes place in two steps with an overall rate of 6 sec À1 . In order to observe catalysis of pseudouridine formation directly, the traditional tritium release assay was adapted for the quench-flow technique, allowing, for the first time, observation of a single round of pseudouridine formation. Thereby, the single-round rate constant of pseudouridylation (k C ) by TruB was determined to be 0.5 sec À1 . This rate constant is similar to the k cat obtained under multiple-turnover conditions in steady-state experiments, indicating that catalysis is the rate-limiting step for TruB. In order to investigate if pseudouridine synthases are characterized by slow catalysis in general, the rapid kinetic quench-flow analysis was also performed with two other E. coli enzymes, RluA and TruA, which displayed rate constants of pseudouridine formation of 0.7 and 0.35 sec À1 , respectively. Hence, uniformly slow catalysis might be a general feature of pseudouridine synthases that share a conserved catalytic domain and supposedly use the same catalytic mechanism.

Cellular RNAs are chemically modified by many RNA modification enzymes; however, often the functions of modifications remain unclear, such as for pseudouridine formation in the tRNA TΨC arm by the bacterial tRNA pseudouridine synthase TruB. Here we test the hypothesis that RNA modification enzymes also act as RNA chaperones. Using TruB as a model, we demonstrate that TruB folds tRNA independent of its catalytic activity, thus increasing the fraction of tRNA that can be aminoacylated. By rapid kinetic stopped-flow analysis, we identified the molecular mechanism of TruB's RNA chaperone activity: TruB binds and unfolds both misfolded and folded tRNAs thereby providing misfolded tRNAs a second chance at folding. Previously, it has been shown that a catalytically inactive TruB variant has no phenotype when expressed in an Escherichia coli truB KO strain [Gutgsell N, et al. (2000) RNA 6(12):1870-1881]. However, here we uncover that E. coli strains expressing a TruB variant impaired in tRNA binding and in in vitro tRNA folding cannot compete with WT E. coli. Consequently, the tRNA chaperone activity of TruB is critical for bacterial fitness. In conclusion, we prove the tRNA chaperone activity of the pseudouridine synthase TruB, reveal its molecular mechanism, and demonstrate its importance for cellular fitness. We discuss the likelihood that other RNA modification enzymes are also RNA chaperones.lthough there is a wealth of information on RNA structure, we are just beginning to understand the RNA folding process that is often assisted by RNA chaperones (1). In contrast to many protein chaperones, RNA chaperones are not ATPases, but instead facilitate unfolding and folding of RNA directly through their interactions with RNA. In addition, the vast majority of all RNAs, including mRNAs, are posttranscriptionally modified by a plethora of RNA modification enzymes (2). Very little is known about the interplay of RNA folding and modification, although it has been speculated that RNA modification enzymes may also act as RNA chaperones (3).Despite the abundance of RNA modifications, their cellular functions are often unclear, including their possible contributions to RNA structure and stability (4). Interestingly, very few RNA modification enzymes are essential for the cell; however, many of these enzymes are conserved. The most abundant RNA modification is the conversion of uridines to pseudouridines that are found in almost all cellular RNAs (4-6). Pseudouridine formation is catalyzed by stand-alone pseudouridine synthases in all domains of life and in addition by H/ACA small ribonucleoproteins (H/ACA sRNPs) in eukaryotes and archaea (7). Remarkably, the only essential pseudouridine synthase is the eukaryotic enzyme Cbf5, the catalytic component of H/ACA sRNPs, whereas all known stand-alone pseudouridine synthases are nonessential (8). Indeed, deletion of most stand-alone pseudouridine synthases in Escherichia coli (9, 10) or Saccharomyces cerevisiae (11) does not impact cell growth under optimal conditions. Surprisingly, ev...

tRNAs are the most highly modified RNAs in all cells, and formation of 5-methyluridine (m5U) at position 54 in the T arm is a common RNA modification found in all tRNAs. The m5U modification is generated by the methyltransferase TrmA. Here, we test and prove the hypothesis that Escherichia coli TrmA has dual functions, acting both as a methyltransferase and as a tRNA chaperone. We identify two conserved residues, F106 and H125, in the RNA-binding domain of TrmA, which interact with the tRNA elbow and are critical for tRNA binding. Co-culture competition assays reveal that the catalytic activity of TrmA is important for cellular fitness, and that substitutions of F106 or H125 impair cellular fitness. We directly show that TrmA enhances tRNA folding in vitro independent of its catalytic activity. In conclusion, our study suggests that F106 and H125 in the RNA-binding domain of TrmA act as a wedge disrupting tertiary interactions between tRNA’s D arm and T arm; this tRNA unfolding is the mechanistic basis for TrmA’s tRNA chaperone activity. TrmA is the second tRNA modifying enzyme next to the pseudouridine synthase TruB shown to act as a tRNA chaperone supporting a functional link between RNA modification and folding.

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