Methylation of adenosine within mRNA coding regions delays tRNA accommodation during translation and thus changes translation dynamics, which might influence protein folding. N6-methylation of adenosine (forming m6A) is the most abundant post-transcriptional modification within the coding region of mRNA, but its role during translation remains unknown. Here, we used bulk kinetic and single-molecule methods to probe the effect of m6A in mRNA decoding. Although m6A base-pairs with uridine during decoding, as shown by X-ray crystallographic analyses of Thermus thermophilus ribosomal complexes, our measurements in an Escherichia coli translation system revealed that m6A modification of mRNA acts as a barrier to tRNA accommodation and translation elongation. The interaction between an m6A-modified codon and cognate tRNA echoes the interaction between a near-cognate codon and tRNA, because delay in tRNA accommodation depends on the position and context of m6A within codons and on the accuracy level of translation. Overall, our results demonstrate that chemical modification of mRNA can change translational dynamics.

During protein synthesis, the ribosome selects aminoacyl-transfer RNAs with anticodons matching the messenger RNA codon present in the A site of the small ribosomal subunit. The aminoglycoside antibiotic streptomycin disrupts decoding by binding close to the site of codon recognition. Here we use X-ray crystallography to define the impact of streptomycin on the decoding site of the Thermus thermophilus 30S ribosomal subunit in complexes with cognate or near-cognate anticodon stem–loop analogues and messenger RNA. Our crystal structures display a significant local distortion of 16S ribosomal RNA induced by streptomycin, including the crucial bases A1492 and A1493 that participate directly in codon recognition. Consistent with kinetic data, we observe that streptomycin stabilizes the near-cognate anticodon stem–loop analogue complex, while destabilizing the cognate anticodon stem–loop analogue complex. These data reveal how streptomycin disrupts the recognition of cognate anticodon stem–loop analogues and yet improves recognition of a near-cognate anticodon stem–loop analogue.

We use extremely bright and ultrashort pulses from an x-ray free-electron laser (XFEL) to measure correlations in x rays scattered from individual bioparticles. This allows us to go beyond the traditional crystallography and single-particle imaging approaches for structure investigations. We employ angular correlations to recover the three-dimensional (3D) structure of nanoscale viruses from x-ray diffraction data measured at the Linac Coherent Light Source. Correlations provide us with a comprehensive structural fingerprint of a 3D virus, which we use both for model-based and ab initio structure recovery. The analyses reveal a clear indication that the structure of the viruses deviates from the expected perfect icosahedral symmetry. Our results anticipate exciting opportunities for XFEL studies of the structure and dynamics of nanoscale objects by means of angular correlations.

A concentric-flow microfluidic electrokinetic sample injector enables efficient delivery of microcrystals in their mother liquor for serial femtosecond X-ray crystallography with minimal sample consumption. We describe a concentric-flow electrokinetic injector for efficiently delivering microcrystals for serial femtosecond X-ray crystallography analysis that enables studies of challenging biological systems in their unadulterated mother liquor. We used the injector to analyze microcrystals of Geobacillus stearothermophilus thermolysin (2.2-Å structure), Thermosynechococcus elongatus photosystem II (<3-Å diffraction) and Thermus thermophilus small ribosomal subunit bound to the antibiotic paromomycin at ambient temperature (3.4-Å structure).

A monthly journal publishing high-quality, peer-reviewed research on all topics related to RNA and its metabolism in all organisms