Molecular biology. A renewed focus on transfer RNA

Protonation of base pairs in RNA: context analysis and quantum chemical investigations of their geometries and stabilities

Base pairs involving protonated nucleobases play important roles in mediating global macromolecular conformational changes and in facilitation of catalysis in a variety of functional RNA molecules. Here we present our attempts at understanding the role of such base pairs by detecting possible protonated base pairs in the available RNA crystal structures using BPFind software, in their specific structural contexts, and by the characterization of their geometries, interaction energies, and stabilities using advanced quantum chemical computations. We report occurrences of 18 distinct protonated base pair combinations from a representative data set of RNA crystal structures and propose a theoretical model for one putative base pair combination. Optimization of base pair geometries was carried out at the B3LYP/cc-pVTZ level, and the BSSE corrected interaction energies were calculated at the MP2/aug-cc-pVDZ level of theory. The geometries for each of the base pairs were characterized in terms of H-bonding patterns observed, rmsd values observed on optimization, and base pair geometrical parameters. In addition, the intermolecular interaction in these complexes was also analyzed using Morokuma energy decomposition. The gas phase interaction energies of the base pairs range from -24 to -49 kcal/mol and reveal the dominance of Hartree-Fock component of interaction energy constituting 73% to 98% of the total interaction energy values. On the basis of our combined bioinformatics and quantum chemical analysis of different protonated base pairs, we suggest resolution of structural ambiguities and correlate their geometric and energetic features with their structural and functional roles. In addition, we also examine the suitability of specific base pairs as key elements in molecular switches and as nucleators for higher order structures such as base triplets and quartets.

The evolution and functional repertoire of translation proteins following the origin of life

Background

The RNA world hypothesis posits that the earliest genetic system consisted of informational RNA molecules that directed the synthesis of modestly functional RNA molecules. Further evidence suggests that it was within this RNA-based genetic system that life developed the ability to synthesize proteins by translating genetic code. Here we investigate the early development of the translation system through an evolutionary survey of protein architectures associated with modern translation.

Results

Our analysis reveals a structural expansion of translation proteins immediately following the RNA world and well before the establishment of the DNA genome. Subsequent functional annotation shows that representatives of the ten most ancestral protein architectures are responsible for all of the core protein functions found in modern translation.

Conclusions

We propose that this early robust translation system evolved by virtue of a positive feedback cycle in which the system was able to create increasingly complex proteins to further enhance its own function.

Reviewers

This article was reviewed by Janet Siefert, George Fox, and Antonio Lazcano (nominated by Laura Landweber)

Colicin E3 cleavage of 16S rRNA impairs decoding and accelerates tRNA translocation on Escherichia coli ribosomes

The cytotoxin colicin E3 targets the 30S subunit of bacterial ribosomes and specifically cleaves 16S rRNA at the decoding centre, thereby inhibiting translation. Although the cleavage site is well known, it is not clear which step of translation is inhibited. We studied the effects of colicin E3 cleavage on ribosome functions by analysing individual steps of protein synthesis. We find that the cleavage affects predominantly the elongation step. The inhibitory effect of colicin E3 cleavage originates from the accumulation of sequential impaired decoding events, each of which results in low occupancy of the A site and, consequently, decreasing yield of elongating peptide. The accumulation leads to an almost complete halt of translation after reading of a few codons. The cleavage of 16S rRNA does not impair monitoring of codon-anticodon complexes or GTPase activation during elongation-factor Tu-dependent binding of aminoacyl-tRNA, but decreases the stability of the codon-recognition complex and slows down aminoacyl-tRNA accommodation in the A site. The tRNA-mRNA translocation is faster on colicin E3-cleaved than on intact ribosomes and is less sensitive to inhibition by the antibiotic viomycin.

Life as a nanoscale phenomenon

The nanoscale is not just the middle ground between molecular and macroscopic but a dimension that is specifically geared to the gathering, processing, and transmission of chemical-based information. Herein we consider the living cell as an integrated self-regulating complex chemical system run principally by nanoscale miniaturization, and propose that this specific level of dimensional constraint is critical for the emergence and sustainability of cellular life in its minimal form. We address key aspects of the structure and function of the cell interface and internal metabolic processing that are coextensive with the up-scaling of molecular components to globular nanoobjects (integral membrane proteins, enzymes, and receptors, etc) and higher-order architectures such as microtubules, ribosomes, and molecular motors. Future developments in nanoscience could provide the basis for artificial life.

Focus on function: single molecule RNA enzymology

The ability of RNA to catalyze chemical reactions was first demonstrated 25 years ago with the discovery that group I introns and RNase P function as RNA enzymes (ribozymes). Several additional ribozymes were subsequently identified, most notably the ribosome, followed by intense mechanistic studies. More recently, the introduction of single molecule tools has dissected the kinetic steps of several ribozymes in unprecedented detail and has revealed surprising heterogeneity not evident from ensemble approaches. Still, many fundamental questions of how RNA enzymes work at the molecular level remain unanswered. This review surveys the current status of our understanding of RNA catalysis at the single molecule level and discusses the existing challenges and opportunities in developing suitable assays.

Conserved loop sequence of helix 69 in Escherichia coli 23 S rRNA is involved in A-site tRNA binding and translational fidelity

Ribosomal (r) RNAs play a crucial role in the fundamental structure and function of the ribosome. Helix 69 (H69) (position 1906-1924), a highly conserved stem-loop in domain IV of the 23 S rRNA of bacterial 50 S subunits, is located on the surface for intersubunit association with the 30 S subunit by connecting with helix 44 of 16 S rRNA with the bridge B2a. H69 directly interacts with A/T-, A-, and P-site tRNAs during each translation step. To investigate the functional importance of the highly conserved loop sequence (1912-1918) of H69, we employed a genetic method that we named SSER (systematic selection of functional sequences by enforced replacement). This method allowed us to identify and select from the randomized loop sequences of H69 in Escherichia coli 23 S rRNA functional sequences that are absolutely required for ribosomal function. From a library consisting of 16,384 sequence variations, 13 functional variants were obtained. A1912 and U(Psi)1917 were selected as essential residues in all variants. An E. coli strain having 23 S rRNA with a U to A mutation at position 1915 showed a severe growth phenotype and low translational fidelity. The result could be explained by the fact that the A1915-ribosome variant has weak subunit association, weak A-site tRNA binding, and decreased translational activity. This study proposes that H69 plays an important role in the control of translational fidelity by modulating A-site tRNA binding during the decoding process.

Identification of functionally important amino acids of ribosomal protein L3 by saturation mutagenesis

There is accumulating evidence that many ribosomal proteins are involved in shaping rRNA into their functionally correct conformations through RNA-protein interactions. Moreover, although rRNA seems to play the central role in all aspects of ribosome function, ribosomal proteins may be involved in facilitating communication between different functional regions in ribosome, as well as between the ribosome and cellular factors. In an effort to more fully understand how ribosomal proteins may influence ribosome function, we undertook large-scale mutational analysis of ribosomal protein L3, a core protein of the large subunit that has been implicated in numerous ribosome-associated functions in the past. A total of 98 different rpl3 alleles were genetically characterized with regard to their effects on killer virus maintenance, programmed -1 ribosomal frameshifting, resistance/hypersensitivity to the translational inhibitor anisomycin and, in specific cases, the ability to enhance translation of a reporter mRNA lacking the 5' (7)mGppp cap structure and 3' poly(A) tail. Biochemical studies reveal a correlation between an increased affinity for aminoacyl-tRNA and the extent of anisomycin resistance and a decreased peptidyltransferase activity and increased frameshifting efficiency. Immunoblot analyses reveal that the superkiller phenotype is not due to a defect in the ability of ribosomes to recruit the Ski-complex, suggesting that the defect lies in a reduced ability of mutant ribosomes to distinguish between cap(+)/poly(A)(+) and cap(-)/poly(A)(-) mRNAs. The results of these analyses are discussed with regard to how protein-rRNA interactions may affect ribosome function.