Domain movements during CCA-addition: a new function for motif C in the catalytic core of the human tRNA nucleotidyltransferases

Substrate Affinity Versus Catalytic Efficiency: Ancestral Sequence Reconstruction of tRNA Nucleotidyltransferases Solves an Enzyme Puzzle

In tRNA maturation, CCA-addition by tRNA nucleotidyltransferase is a unique and highly accurate reaction. While the mechanism of nucleotide selection and polymerization is well understood, it remains a mystery why bacterial and eukaryotic enzymes exhibit an unexpected and surprisingly low tRNA substrate affinity while they efficiently catalyze the CCA-addition. To get insights into the evolution of this high-fidelity RNA synthesis, the reconstruction and characterization of ancestral enzymes is a versatile tool. Here, we investigate a reconstructed candidate of a 2 billion years old CCA-adding enzyme from Gammaproteobacteria and compare it to the corresponding modern enzyme of Escherichia coli. We show that the ancestral candidate catalyzes an error-free CCA-addition, but has a much higher tRNA affinity compared with the extant enzyme. The consequence of this increased substrate binding is an enhanced reverse reaction, where the enzyme removes the CCA end from the mature tRNA. As a result, the ancestral candidate exhibits a lower catalytic efficiency in vitro as well as in vivo. Furthermore, the efficient tRNA interaction leads to a processive polymerization, while the extant enzyme catalyzes nucleotide addition in a distributive way. Thus, the modern enzymes increased their polymerization efficiency by lowering the binding affinity to tRNA, so that CCA synthesis is efficiently promoted due to a reduced reverse reaction. Hence, the puzzling and at a first glance contradicting and detrimental weak substrate interaction represents a distinct activity enhancement in the evolution of CCA-adding enzymes.

CCA-addition in the cold: Structural characterization of the psychrophilic CCA-adding enzyme from the permafrost bacterium Planococcus halocryophilus

•

A high-resolution structure of a psychrophilic RNA polymerase contributes to our knowledge of cold adaptation.

•

While catalytic core motifs are conserved, at least one shows cold adaptation.

•

Loss of helix-capping increases structural flexibility in a catalytic core motif.

•

Overall reduction of alpha-helical elements appears as a strategy for cold adaptation.

CCA-adding enzymes are highly specific RNA polymerases that add and maintain the sequence C-C-A at tRNA 3‘-ends. Recently, we could reveal that cold adaptation of such a polymerase is not only achieved at the expense of enzyme stability, but also at the cost of polymerization fidelity. Enzymes from psychrophilic organisms usually show an increased structural flexibility to enable catalysis at low temperatures. Here, polymerases face a dilemma, as there is a discrepancy between the need for a tightly controlled flexibility during polymerization and an increased flexibility as strategy for cold adaptation. Based on structural and biochemical analyses, we contribute to clarify the cold adaptation strategy of the psychrophilic CCA-adding enzyme from Planococcus halocryophilus, a gram-positive bacterium thriving in the arctic permafrost at low temperatures down to −15 °C. A comparison with the closely related enzyme from the thermophilic bacterium Geobacillus stearothermophilus reveals several features of cold adaptation - a significantly reduced amount of alpha-helical elements in the C-terminal tRNA-binding region and a structural adaptation in one of the highly conserved catalytic core motifs located in the N-terminal catalytic core of the enzyme.

CCA-Addition Gone Wild: Unusual Occurrence and Phylogeny of Four Different tRNA Nucleotidyltransferases in Acanthamoeba castellanii

tRNAs are important players in the protein synthesis machinery, where they act as adapter molecules for translating the mRNA codons into the corresponding amino acid sequence. In a series of highly conserved maturation steps, the primary transcripts are converted into mature tRNAs. In the amoebozoan Acanthamoeba castellanii, a highly unusual evolution of some of these processing steps was identified that are based on unconventional RNA polymerase activities. In this context, we investigated the synthesis of the 3'-terminal CCA-end that is added posttranscriptionally by a specialized polymerase, the tRNA nucleotidyltransferase (CCA-adding enzyme). The majority of eukaryotic organisms carry only a single gene for a CCA-adding enzyme that acts on both the cytosolic and the mitochondrial tRNA pool. In a bioinformatic analysis of the genome of this organism, we identified a surprising multitude of genes for enzymes that contain the active site signature of eukaryotic/eubacterial tRNA nucleotidyltransferases. In vitro activity analyses of these enzymes revealed that two proteins represent bona fide CCA-adding enzymes, one of them carrying an N-terminal sequence corresponding to a putative mitochondrial target signal. The other enzymes have restricted activities and represent CC- and A-adding enzymes, respectively. The A-adding enzyme is of particular interest, as its sequence is closely related to corresponding enzymes from Proteobacteria, indicating a horizontal gene transfer. Interestingly, this unusual diversity of nucleotidyltransferase genes is not restricted to Acanthamoeba castellanii but is also present in other members of the Acanthamoeba genus, indicating an ancient evolutionary trait.

Adaptation of the Romanomermis culicivorax CCA-Adding Enzyme to Miniaturized Armless tRNA Substrates

The mitochondrial genome of the nematode Romanomermis culicivorax encodes for miniaturized hairpin-like tRNA molecules that lack D- as well as T-arms, strongly deviating from the consensus cloverleaf. The single tRNA nucleotidyltransferase of this organism is fully active on armless tRNAs, while the human counterpart is not able to add a complete CCA-end. Transplanting single regions of the Romanomermis enzyme into the human counterpart, we identified a beta-turn element of the catalytic core that-when inserted into the human enzyme-confers full CCA-adding activity on armless tRNAs. This region, originally identified to position the 3'-end of the tRNA primer in the catalytic core, dramatically increases the enzyme's substrate affinity. While conventional tRNA substrates bind to the enzyme by interactions with the T-arm, this is not possible in the case of armless tRNAs, and the strong contribution of the beta-turn compensates for an otherwise too weak interaction required for the addition of a complete CCA-terminus. This compensation demonstrates the remarkable evolutionary plasticity of the catalytic core elements of this enzyme to adapt to unconventional tRNA substrates.

Unusual Occurrence of Two Bona-Fide CCA-Adding Enzymes in Dictyostelium discoideum

Dictyostelium discoideum, the model organism for the evolutionary supergroup of Amoebozoa, is a social amoeba that, upon starvation, undergoes transition from a unicellular to a multicellular organism. In its genome, we identified two genes encoding for tRNA nucleotidyltransferases. Such pairs of tRNA nucleotidyltransferases usually represent collaborating partial activities catalyzing CC- and A-addition to the tRNA 3'-end, respectively. In D. discoideum, however, both enzymes exhibit identical activities, representing bona-fide CCA-adding enzymes. Detailed characterization of the corresponding activities revealed that both enzymes seem to be essential and are regulated inversely during different developmental stages of D. discoideum. Intriguingly, this is the first description of two functionally equivalent CCA-adding enzymes using the same set of tRNAs and showing a similar distribution within the cell. This situation seems to be a common feature in Dictyostelia, as other members of this phylum carry similar pairs of tRNA nucleotidyltransferase genes in their genome.

Divergent Evolution of Eukaryotic CC- and A-Adding Enzymes

Synthesis of the CCA end of essential tRNAs is performed either by CCA-adding enzymes or as a collaboration between enzymes restricted to CC- and A-incorporation. While the occurrence of such tRNA nucleotidyltransferases with partial activities seemed to be restricted to Bacteria, the first example of such split CCA-adding activities was reported in Schizosaccharomyces pombe. Here, we demonstrate that the choanoflagellate Salpingoeca rosetta also carries CC- and A-adding enzymes. However, these enzymes have distinct evolutionary origins. Furthermore, the restricted activity of the eukaryotic CC-adding enzymes has evolved in a different way compared to their bacterial counterparts. Yet, the molecular basis is very similar, as highly conserved positions within a catalytically important flexible loop region are missing in the CC-adding enzymes. For both the CC-adding enzymes from S. rosetta as well as S. pombe, introduction of the loop elements from closely related enzymes with full activity was able to restore CCA-addition, corroborating the significance of this loop in the evolution of bacterial as well as eukaryotic tRNA nucleotidyltransferases. Our data demonstrate that partial CC- and A-adding activities in Bacteria and Eukaryotes are based on the same mechanistic principles but, surprisingly, originate from different evolutionary events.

Analysis of the pathogenic I326T variant of human tRNA nucleotidyltransferase reveals reduced catalytic activity and thermal stability in vitro linked to a conformational change

The I326T mutation in the TRNT1 gene encoding human tRNA nucleotidyltransferase (tRNA-NT) is linked to a relatively mild form of SIFD. Previous work indicated that the I326T variant was unable to incorporate AMP into tRNAs in vitro, however, expression of the mutant allele from a strong heterologous promoter supported in vivo CCA addition to both cytosolic and mitochondrial tRNAs in a yeast strain lacking tRNA-NT. To address this discrepancy, we determined the biochemical and biophysical characteristics of the I326T variant enzyme and the related variant, I326A. Our in vitro analysis revealed that the I326T substitution decreases the thermal stability of the enzyme and causes a ten-fold reduction in enzyme activity. We propose that the structural changes in the I326T variant that lead to these altered parameters result from a rearrangement of helices within the body domain of the protein which can be probed by the inability of the monomeric enzyme to form a covalent dimer in vitro mediated by C373. In addition, we confirm that the effects of the I326T or I326A substitutions are relatively mild in vivo by demonstrating that the mutant alleles support both mitochondrial and cytosolic CCA-addition in yeast.

Combining crystallogenesis methods to produce diffraction-quality crystals of a psychrophilic tRNA-maturation enzyme

The determination of conditions for the reproducible growth of well diffracting crystals is a critical step in every biocrystallographic study. On the occasion of a new structural biology project, several advanced crystallogenesis approaches were tested in order to increase the success rate of crystallization. These methods included screening by microseed matrix screening, optimization by counter-diffusion and crystal detection by trace fluorescent labeling, and are easily accessible to any laboratory. Their combination proved to be particularly efficient in the case of the target, a 48 kDa CCA-adding enzyme from the psychrophilic bacterium Planococcus halocryophilus. A workflow summarizes the overall strategy, which led to the production of crystals that diffracted to better than 2 Å resolution and may be of general interest for a variety of applications.

In vitro studies of disease-linked variants of human tRNA nucleotidyltransferase reveal decreased thermal stability and altered catalytic activity

Mutations in the human TRNT1 gene encoding tRNA nucleotidyltransferase (tRNA-NT), an essential enzyme responsible for addition of the CCA (cytidine-cytidine-adenosine) sequence to the 3'-termini of tRNAs, have been linked to disease phenotypes including congenital sideroblastic anemia with B-cell immunodeficiency, periodic fevers and developmental delay (SIFD) or retinitis pigmentosa with erythrocyte microcytosis. The effects of these disease-linked mutations on the structure and function of tRNA-NT have not been explored. Here we use biochemical and biophysical approaches to study how five SIFD-linked amino acid substitutions (T154I, M158V, L166S, R190I and I223T), residing in the N-terminal head and neck domains of the enzyme, affect the structure and activity of human tRNA-NT in vitro. Our data suggest that the SIFD phenotype is linked to poor stability of the T154I and L166S variant proteins, and to a combination of reduced stability and altered catalytic efficiency in the M158 V, R190I and I223T variants.

A tRNA's fate is decided at its 3' end: Collaborative actions of CCA-adding enzyme and RNases involved in tRNA processing and degradation

tRNAs are key players in translation and are additionally involved in a wide range of distinct cellular processes. The vital importance of tRNAs becomes evident in numerous diseases that are linked to defective tRNA molecules. It is therefore not surprising that the structural intactness of tRNAs is continuously scrutinized and defective tRNAs are eliminated. In this process, erroneous tRNAs are tagged with single-stranded RNA sequences that are recognized by degrading exonucleases. Recent discoveries have revealed that the CCA-adding enzyme - actually responsible for the de novo synthesis of the 3'-CCA end - plays an indispensable role in tRNA quality control by incorporating a second CCA triplet that is recognized as a degradation tag. In this review, we give an update on the latest findings regarding tRNA quality control that turns out to represent an interplay of the CCA-adding enzyme and RNases involved in tRNA degradation and maturation. In particular, the RNase-induced turnover of the CCA end is now recognized as a trigger for the CCA-adding enzyme to repeatedly scrutinize the structural intactness of a tRNA. This article is part of a Special Issue entitled: SI: Regulation of tRNA synthesis and modification in physiological conditions and disease edited by Dr. Boguta Magdalena.

Cold adaptation of tRNA nucleotidyltransferases: A tradeoff in activity, stability and fidelity

Cold adaptation is an evolutionary process that has dramatic impact on enzymatic activity. Increased flexibility of the protein structure represents the main evolutionary strategy for efficient catalysis and reaction rates in the cold, but is achieved at the expense of structural stability. This results in a significant activity-stability tradeoff, as it was observed for several metabolic enzymes. In polymerases, however, not only reaction rates, but also fidelity plays an important role, as these enzymes have to synthesize copies of DNA and RNA as exact as possible. Here, we investigate the effects of cold adaptation on the highly accurate CCA-adding enzyme, an RNA polymerase that uses an internal amino acid motif within the flexible catalytic core as a template to synthesize the CCA triplet at tRNA 3'-ends. As the relative orientation of these residues determines nucleotide selection, we characterized how cold adaptation impacts template reading and fidelity. In a comparative analysis of closely related psychro-, meso-, and thermophilic enzymes, the cold-adapted polymerase shows a remarkable error rate during CCA synthesis in vitro as well as in vivo. Accordingly, CCA-adding activity at low temperatures is not only achieved at the expense of structural stability, but also results in a reduced polymerization fidelity.