Comprehensive Characterization of tRNA by Intact Mass Analysis

Significance of the topic

Transfer RNA (tRNA) molecules carry a high density of post-transcriptional modifications and undergo stepwise maturation that are critical for accurate translation and cellular regulation. Intact-mass analysis of tRNA provides a rapid, complementary approach to conventional LC-MS/MS mapping of enzymatic digests: it can detect full-length variants, truncations, post-transcriptional processing states and certain modification intermediates directly, enabling characterization of isoforms, maturation intermediates and modification heterogeneity that are important for basic research, quality control of RNA preparations, and studies of RNA biology and therapeutics.

Objectives and overview of the study

• Demonstrate that high-resolution intact-mass analysis (UHPLC-HRAM-MS) can resolve and identify intact tRNA species and their variants by deconvolution of negative charge-state envelopes.
• Apply the workflow to a commercial Saccharomyces cerevisiae tRNAPhe standard to: identify isodecoders and sequence variants, detect 3'-end maturation states (truncations and intermediates), and discover modification intermediates in the wybutosine (yW) biosynthetic pathway.
• Validate intact-mass based identifications with orthogonal nucleoside and oligonucleotide mass-mapping experiments.

Methodology

• Overall strategy: Ion-pair reversed-phase UHPLC separation of intact tRNA followed by negative-mode high-resolution accurate-mass (HRAM) Orbitrap analysis. Deconvolution of multiply charged spectra provided monoisotopic intact masses, which were matched to expected sequences and modification states. Confirmation was performed using nucleoside-level and RNase-digest oligonucleotide mass mapping and MS/MS.
• Samples: Commercial S. cerevisiae tRNAPhe standard.
• Enzymatic digests:
  • Nucleoside preparation: heat denaturation followed by P1 nuclease, RNase A (trace), and phosphatase digestion (37 °C, 2 h); dried and reconstituted for nucleoside LC-MS.
  • Oligonucleotide mapping: RNase T1 digestion in 200 mM ammonium acetate with 50 U RNase T1 per µg RNA at 40 °C for 2 h; dry down and reconstitute for oligonucleotide LC-MS.
• Chromatography:
  • Intact/oligonucleotide separations: Thermo Scientific DNAPac RP (2.1 x 100 mm) on Vanquish Horizon UHPLC using ion-pairing mobile phases; intact gradient: 10% B to 40% B (0–7 min) to 90% B (7.1–7.6 min), return to 10% B by 7.51 min; flow 400 µL/min; column at 80 °C.
  • Nucleoside separations: Accucore C18+ (2.1 x 100 mm, 1.5 µm) on Vanquish Flex using ammonium acetate buffers; nucleoside gradient: 0% to 25% B (0–10 min) to 99% B (14–15 min).
• Mass spectrometry & data processing:
  • Instrument: Thermo Scientific Orbitrap Ascend Tribrid with heated-ESI (H-ESI), negative polarity.
  • Acquisition: Intact Protein mode, resolution 240,000 at m/z 400, m/z range 800–3000, AGC target 7.5e4, max IT 100 ms, quadrupole isolation 1.2 m/z, RF 35%, ion transfer tube 350 °C, vaporizer 320 °C, spray voltage 3500 V.
  • Deconvolution: Thermo Scientific BioPharma Finder 5.1 (Xtract algorithm). Parameters: TIC trace, m/z 800–3000, averaged spectra over selected time windows; output mass range 24,000–26,000 Da; S/N threshold 3, relative abundance threshold 3%; charge range -5 to -35 (minimum 5 detected charges); nucleotide table, negative charge. Sequence matching tolerance 10 ppm (multiconsensus merge 10 ppm).
  • MS/MS: CID and HCD fragmentation used for confirmation of nucleoside and oligonucleotide identifications.

Main results and discussion

• Charge-state distribution and deconvolution: The intact tRNAPhe produced a clear negative charge envelope from roughly -9 to -26, with dominant states between -15 and -19. Baseline isotopic resolution—notably of the -17 charge state—facilitated confident deconvolution to monoisotopic masses (example major mass 24610.450 Da).
• Variant detection: Deconvolution returned multiple mass species corresponding to three tRNAPhe variants. Both known isodecoders were identified, as well as a sequence variant at position 68 (A68) exhibiting loss of a methyl group (indicative of loss on the yW side chain).
• 3'-end maturation states: The predominant form detected for all three variants was a 3'-truncated CpCp terminus rather than the mature CCA. Lower-abundance species included CpCpAp with a phosphorylated 3' adenosine, consistent with an intermediate in 3'-end processing or aminoacylation pathways. Observed mass discrepancies relative to theoretical masses were ≤ 4–5 ppm.
• Wybutosine pathway intermediate: A prominent deconvolution peak was consistent with a demethylated form of the yW side chain (yW-14 intermediate). Nucleoside-level EICs for the expected protonated mass showed a chromatographic feature between known yW precursors; CID and HCD MS/MS fragmentation patterns matched the predicted yW-14 structure, supporting assignment of a biosynthetic intermediate.
• Orthogonal confirmation: RNase-digest mass mapping and nucleoside MS/MS corroborated the intact-mass based assignments, including fragments diagnostic for yW-14–containing oligonucleotides and typical CID RNA ladder ions.

Benefits and practical applications of the method

• Fast detection of intact tRNA molecular species, enabling observation of truncations, sequence variants and modification heterogeneity in a single analysis.
• High mass accuracy and isotopic resolution allow discrimination of closely related modification states and detection of low-mass deltas (≤ 5 ppm), useful for identifying biosynthetic intermediates.
• Complementary to digestion-based LC-MS/MS workflows: intact-mass deconvolution directs targeted follow-up (nucleoside/oligonucleotide MS/MS), reducing time to identification.
• Applications include RNA quality assessment for research and therapeutic production, studies of tRNA maturation and modification pathways, and discovery of novel intermediates or under-modified isoforms.

Instrumentation used

• UHPLC systems: Thermo Scientific Vanquish Horizon Quaternary UHPLC and Vanquish Flex UHPLC.
• Columns: Thermo Scientific DNAPac RP (4 µm, 2.1 × 100 mm) for intact/oligonucleotide separations; Thermo Scientific Accucore C18+ (1.5 µm, 2.1 × 100 mm) for nucleoside separations.
• Mass spectrometer: Thermo Scientific Orbitrap Ascend Tribrid with heated-ESI source.
• Software: Thermo Scientific Xcalibur 4.5, Freestyle 1.8, and BioPharma Finder 5.1 (Intact Mass Analysis).

Future trends and potential applications

• Higher-resolution and native MS approaches could preserve noncovalent structure and complexes, enabling direct analysis of tRNA-bound proteins or aaRS interactions.
• Improvements in data analysis and targeted deconvolution algorithms will increase sensitivity for low-abundance isoforms and complex modification patterns.
• Integration with top-down sequencing approaches and advanced fragmentation techniques (ETD/AI-ETD for nucleic acids) could provide direct sequence-level localization of modifications on intact RNAs.
• Application to longer RNAs and structured noncoding RNAs as ionization and chromatography methods advance, expanding utility in RNA therapeutics and quality control of synthetic RNAs.
• Standardization of workflows and reference materials will facilitate routine use in industrial QC and regulatory environments.

Conclusion

This study demonstrates that ion-pair reversed-phase UHPLC coupled to high-resolution Orbitrap mass spectrometry and robust deconvolution software can characterize intact tRNA species with sufficient mass accuracy and isotopic resolution to resolve isodecoders, sequence variants, 3'-end maturation states and biosynthetic modification intermediates. When combined with nucleoside and oligonucleotide MS/MS validation, the approach provides a powerful, complementary toolkit for in-depth tRNA characterization relevant to research and applied settings.

Reference

1. Keith G, Dirheimer G. Evidence for the existence of an expressed minor variant tRNAPhe in yeast. Biochem Biophys Res Commun. 1987 Jan 15;142(1):183-187.
2. Noma A, et al. Biosynthesis of wybutosine, a hyper-modified nucleoside in eukaryotic phenylalanine tRNA. EMBO J. 2006 May 17;25(10):2142-2154.