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Sarcin-ricin loop with modified residue.
Structural highlights
Publication Abstract from PubMedGNRA (N = A, C, G, or U; R = A or G) tetraloops are common RNA secondary structural motifs and feature a phosphate stacked atop a nucleobase. The rRNA sarcin/ricin loop (SRL) is capped by GApGA, and the phosphate p stacks on G. We recently found that regiospecific incorporation of a single dithiophosphate (PS2) but not a monothiophosphate (PSO) instead of phosphate in the backbone of RNA aptamers dramatically increases the binding affinity for their targets. In the RNA:thrombin complex, the key contribution to the 1000-fold tighter binding stems from an edge-on contact between PS2 and a phenylalanine ring. Here we investigated the consequences of replacing the SRL phosphate engaged in a face-on interaction with guanine with either PS2 or PSO for stability. We found that PS2...G and Rp-PSO...G contacts stabilize modified SRLs compared to the parent loop to unexpected levels: up to 6.3 degrees C in melting temperature Tm and -4.7 kcal/mol in DeltaDeltaG degrees . Crystal structures demonstrate that the vertical distance to guanine for the closest sulfur is just 0.05 A longer on average compared to that of oxygen despite the larger van der Waals radius of the former (1.80 A for S vs 1.52 A for O). The higher stability is enthalpy-based, and the negative charge as assessed by a neutral methylphosphonate modification plays only a minor role. Quantum mechanical/molecular mechanical calculations are supportive of favorable dispersion attraction interactions by sulfur making the dominant contribution. A stacking interaction between phosphate and guanine (SRL) or uracil (U-turn) is also found in newly classified RNA tetraloop families besides GNRA. Incorporating a Thiophosphate Modification into a Common RNA Tetraloop Motif Causes an Unanticipated Stability Boost.,Pallan PS, Lybrand TP, Schlegel MK, Harp JM, Jahns H, Manoharan M, Egli M Biochemistry. 2020 Dec 15;59(49):4627-4637. doi: 10.1021/acs.biochem.0c00685., Epub 2020 Dec 4. PMID:33275419[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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