Substrate binding and catalysis by the pseudouridine synthases RluA and TruB

xi, 122 leaves : ill. (some col.) ; 29 cmPseudouridine is the most common RNA modification found in all forms of life. The exact role pseudouridines play in the cell is still relatively unknown. However, its extensive incorporation in functionally important areas of the ribosome and the fitness advantage provided to cells by pseudouridines implies that its presence is important for the cell. The enzymes responsible for this modification, pseudouridine synthases, vary greatly in substrate recognition mechanisms, but all enzymes supposedly share a universally conserved catalytic mechanism. Here, I analyze the kinetic mechanisms of pseudouridylation utilized by the exemplary pseudouridine synthase RluA in order to compare it with the previously determined rate of pseudouridylation of the pseudouridine synthase TruB. My results demonstrate that RluA has the same uniformly slow catalytic step as previously determined for TruB and TruA. This constitutes the first step towards identifying the catalytic mechanism of the pseudouridine synthase family. Additionally, it was my aim to identify the major determinants for RNA binding by pseudouridine synthases. By measuring the dissociation constants (KD) for substrate and product tRNA by nitrocellulose filtration assays, I showed that both tRNA species could bind with similar affinities. These binding studies also revealed that TruB’s interaction with the isolated T-arm is the major contact site contributing to the affinity of the enzyme to RNA. Finally, a new contact between tRNA and TruB’s PUA domain was identified which was not observed in the crystal structure. In summary, my results provide new insight into the common catalytic step of pseudouridine synthases and the specific interactions contributing to substrate binding by the enzyme TruB. These results will enable future studies on the kinetic mechanism of pseudouridine synthases, in particular the kinetics of substrate and product binding and release, as well as on the chemical mechanism of pseudouridine formation

Functional and mechanistic characterization of two tRNA modifying enzymes

The formation of pseudouridine and 5-methyluridine (m5U) in the T-arm of transfer RNAs (tRNAs) is near-universally conserved. These two modifications are formed in Escherichia coli by the pseudouridine synthase TruB and the S-adenosylmethionine-dependent methyltransferase TrmA, respectively. In this thesis, I investigate the function and mechanisms of these two tRNA modifying enzymes. First, in vitro and in vivo analysis of TruB reveals that this enzyme is acting as a tRNA chaperone which proves a long outstanding hypothesis. Secondly, characterization of ligand binding by TrmA shows that binding is cooperative and disruption of tRNA elbow region tertiary interactions by TrmA is essential for efficient tRNA binding and catalysis, leading to future analysis. In conclusion, my studies further our understanding of the mechanism and function of tRNA modifications and modifying enzymes, as well as shed light on why all cells invest substantial resources into fine-tuning the chemical composition of tRNAs