Conformational preferences of anticodon 3'-adjacent hypermodified nucleic acid base cis-or trans-zeatin and its 2-methylthio derivative, cis-or trans- ms(2)zeatin

Role of Wybutosine and Mg2+ Ions in Modulating the Structure and Function of tRNAPhe: A Molecular Dynamics Study

Transfer RNA remains to be a mysterious molecule of the cell repertoire. With its modified bases and selectivity of codon recognition, it remains to be flexible inside the ribosomal machinery for smooth and hassle-free protein biosynthesis. Structural changes occurring in tRNA due to the presence or absence of wybutosine, with and without Mg²⁺ ions, have remained a point of interest for structural biologists. Very few studies have come to a conclusion correlating the changes either with the structure and flexibility or with the codon recognition. Considering the above facts, we have implemented molecular modeling methods to address these problems using multiple molecular dynamics (MD) simulations of tRNA^Phe along with codons. Our results highlight some of the earlier findings and also shed light on some novel structural and functional aspects. Changes in the stability of tRNA^Phe in native or codon-bound states result from the conformations of constituent nucleotides with respect to each other. A smaller change in their conformations leads to structural distortions in the base-pairing geometry and eventually in the ribose-phosphate backbone. MD simulation studies highlight the preference of UUC codons over UUU by tRNA^Phe in the presence of wybutosine and Mg²⁺ ions. This study also suggests that magnesium ions are required by tRNA^Phe for proper recognition of UUC/UUU codons during ribosomal interactions with tRNA.

Conformational preferences and structural analysis of hypermodified nucleoside, peroxywybutosine (o2yW) found at 37th position in anticodon loop of tRNAPhe and its role in modulating UUC codon-anticodon interactions

Hypermodified bases present at 3'-adjacent (37^th) position in anticodon loop of tRNA^Phe are well known for their contribution in modulating codon-anticodon interactions. Peroxywybutosine (o2yW), a wyosine family member, is one of such tricyclic modified bases observed at the 37^th position in tRNA^Phe. Conformational preferences and three-dimensional structural analysis of peroxywybutosine have not been investigated in detail at atomic level. Hence, in the present study quantum chemical semi-empirical RM1 and multiple molecular dynamics (MD) simulations have been used to study structural significance of peroxywybutosine in tRNA^Phe. Full geometry optimizations over the peroxywybutosine base have also been performed using ab-initio HF-SCF (6-31G**), DFT (B3LYP/6-31G**) and semi-empirical PM6 method to compare the salient properties. RM1 predicted most stable structure shows that the amino-carboxy-propyl side chain of o2yW remains 'distal' to the five membered imidazole ring of tricyclic guanosine. MD simulation trajectory of the isolated peroxy base showed restricted periodical fluctuations of peroxywybutosine side chain which might be helpful to maintain proper anticodon loop structure and mRNA reading frame during protein biosynthesis process. Another comparative MD simulation study of the anticodon stem loop with codon UUC showed various properties, which justify the functional implications of peroxywybutosine at 37^th position along with other modified bases present in ASL of tRNA^Phe. Thus, this study presents an atomic view into the structural properties of peroxywybutosine, which can be useful to determine its role in the anticodon stem loop in context of codon-anticodon interactions and frame shift mutations.

Conformational Preferences of Modified Nucleoside 5-Taurinomethyluridine, τm(5)U Occur at 'wobble' 34th Position in the Anticodon Loop of tRNA

Conformational preferences of hypermodified nucleoside 5-taurinomethyluridine 5'-monophoshate 'p-τm(5)U' (-CH2-NH2(+)-CH2-CH2-SO3(-)) have been investigated using semi-empirical RM1 method. Automated geometry optimization using ab initio molecular orbital HF-SCF (6-31G**) and DFT (B3LYP/6-31G**) calculations have also been made to compare the salient features. The RM1 preferred most stable conformation of 'p-τm(5)U' has been stabilized by hydrogen bonding interactions between O(11a)…HN(8), O1P(34)…HN(8), and O1P(34)…HC(10). Another conformational study of 5-taurinomethyluridine side chain has also been performed in context of anticodon loop bases of E. coli tRNA(Leu). The atom O(11a) of τm(5)U(34) side chain interacts with adenosine (A35) as well as ribose-phosphate backbone which might provide structural stability to the anticodon loop. The glycosyl torsion angle of τm(5)U retains 'anti'-conformation. The solvent accessible surface area calculations revealed the role of τm(5)U in tRNA(Leu) anticodon loop. MD simulation results are found in agreement with RM1 preferred stable structure. The MEPs calculations of τm(5)U(34):G3 model show unique potential tunnels between the hydrogen bond donor and acceptor atoms as compared to τm(5)U(34):A3 model. Thus, these results could pave the way to understand the role of τm(5)U(34) to recognize UUG/UUA codons at atomic level in the mitochondrial disease, MELAS.

The influence of hypermodified nucleosides lysidine and t(6)A to recognize the AUA codon instead of AUG: a molecular dynamics simulation study

Hypermodified nucleosides lysidine (L) and N(6)-threonylcarbamoyladenosine (t(6)A) influence codon-anticodon interactions during the protein biosynthesis process. Lysidine prevents the misrecognition of the AUG codon as isoleucine and that of AUA as methionine. The structural significance of these modified bases has not been studied in detail at the atomic level. Hence, in the present study we performed multiple molecular dynamics (MD) simulations of anticodon stem loop (ASL) of tRNA(Ile) in the presence and absence of modified bases 'L' and 't(6)A' at the 34th and 37th positions respectively along with trinucleotide 'AUA' and 'AUG' codons. Hydrogen bonding interactions formed by the tautomeric form of lysidine may assist in reading the third base adenine of the 'AUA' codon, unlike the guanine of the 'AUG' codon. Such interactions might be useful to restrict codon specificity to recognize isoleucine tRNA instead of methionine tRNA. The t(6)A side chain interacts with the purine ring of the first codon nucleotide adenine, which might provide base stacking interactions and could be responsible for restricting extended codon-anticodon recognition. We found that ASL tRNA(Ile) in the absence of modifications at the 34th and 37th positions cannot establish proper hydrogen bonding interactions to recognize the isoleucine codon 'AUA' and subsequently disturbs the anticodon loop structure. The binding free energy calculations revealed that tRNA(Ile) ASL with modified nucleosides prefers the codon AUA over AUG. Thus, these findings might be useful to understand the role of modified bases L and t(6)A to recognize the AUA codon instead of AUG.

Conformational preferences of modified nucleoside N(4)-acetylcytidine, ac4C occur at "wobble" 34th position in the anticodon loop of tRNA

Conformational preferences of modified nucleoside, N(4)-acetylcytidine, ac(4)C have been investigated using quantum chemical semi-empirical RM1 method. Automated geometry optimization using PM3 method along with ab initio methods HF SCF (6-31G**), and density functional theory (DFT; B3LYP/6-31G**) have also been made to compare the salient features. The most stable conformation of N(4)-acetyl group of ac(4)C prefers "proximal" orientation. This conformation is stabilized by intramolecular hydrogen bonding between O(7)···HC(5), O(2)···HC2', and O4'···HC(6). The "proximal" conformation of N(4)-acetyl group has also been observed in another conformational study of anticodon loop of E. coli elongator tRNA(Met). The solvent accessible surface area (SASA) calculations revealed the role of ac(4)C in anticodon loop. The explicit molecular dynamics simulation study also shows the "proximal" orientation of N(4)-acetyl group. The predicted "proximal" conformation would allow ac(4)C to interact with third base of codon AUG/AUA whereas the 'distal' orientation of N(4)-acetyl cytidine side-chain prevents such interactions. Single point energy calculation studies of various models of anticodon-codon bases revealed that the models ac(4)C(34)(Proximal):G3, and ac(4)C(34)(Proximal):A3 are energetically more stable as compared to models ac(4)C(34)(Distal):G3, and ac(4)C(34)(Distal):A3, respectively. MEPs calculations showed the unique potential tunnels between the hydrogen bond donor-acceptor atoms of ac(4)C(34)(Proximal):G3/A3 base pairs suggesting role of ac(4)C in recognition of third letter of codons AUG/AUA. The "distal" conformation of ac(4)C might prevent misreading of AUA codon. Hence, this study could be useful to understand the role of ac(4)C in the tertiary structure folding of tRNA as well as in the proper recognition of codons during protein biosynthesis process.

MD simulation studies to investigate iso-energetic conformational behaviour of modified nucleosides m(2)G and m(2) 2G present in tRNA

Modified nucleic acid bases are most commonly found in tRNA. These may contain modifications from simple methylation to addition of bulky groups. Methylation of the four canonical nucleotide bases at a wide variety of positions is particularly prominent among the known modification. Methylation of N2 group of guanine is a relatively common modification in tRNA and rRNA. N2-methylguanosine (m²G) is the second most often encountered nucleoside in E. coli tRNAs. N2, N2- dimethylguanosine (m²₂G) is found in the majority of eukaryotic tRNAs and involved in forming base pair interactions with adjacent bases. Hence, in order to understand the structural significance of these methylated nucleic acid bases we have carried out molecular dynamics simulation to see the salvation effect. The results obtained shows iso-energetic conformational behaviors for m²G and m²₂G. The simulation trajectory of m²G shows regular periodical fluctuations suggesting that m²G is equally stable as either s-cis or s-trans rotamers. The two rotamers of m²G may interact canonically or non-canonically with opposite base as s-trans m²G26:C/A/U44 and s-cis m²G26:A/U44. The free rotations around the C-N bond could be the possible reason for these iso-energetic conformations. Dimethylation of G has almost no influence on base pairing with either A or U. Thus, these results reveal that modified nucleosides m²G and m²₂G may play an important role to prevent tRNA from adopting the unusual mitochondrial like conformation.

Structural significance of hypermodified nucleic acid base hydroxywybutine (OHyW) which occur at 37th position in the anticodon loop of yeast tRNA(Phe)

Conformational preferences of hypermodified nucleic acid base hydroxywybutine (OHyW) have been studied using quantum chemical single point semi-empirical PM3 method. Automated geometry optimization using semi-empirical RM1, molecular mechanics force field (MMFF) along with ab-initio HF-SCF (6-31G** basis set) and DFT (B3LYP/6-31G** basis set) calculations have also been made to compare the salient features. Molecular electrostatic potentials (MEPs) depict the polarities of hydroxywybutine (OHyW) side chain. Another conformational study showed that hydroxywybutosine side chain interacts with adjacent bases within the anticodon loop of tRNA(Phe). The solvent accessible surface area (SASA) calculations revealed the structural role of hydroxywybutine in anticodon loop. Explicit molecular dynamics (MD) simulation has been done over the PM3 most stable structure of OHyW. The hydroxywybutine side chain prefers 'distal' conformation i.e. spreads away from the cyclic five membered imidazole moiety of modified tricyclic guanine base. The predicted preferred conformation of hydroxywybutine may prevent extended Watson-Crick base pairing during protein biosynthesis process. This conformation of OHyW stabilized by intramolecular interactions between O(6)⋯HO(16), O(6)⋯HC(15) and O(20)⋯HC(17). Further stabilization is also expected from interactions between O(22)⋯HC(16) and O(23)⋯HC(15). Explicit molecular dynamics (MD) simulation over the PM3 most stable structure of OHyW support the preferred geometry by preserving the 'distal' orientation of hydroxywybutine side chain and intramolecular hydrogen bonding interactions. MD simulation study revealed the role of hydroxyl group of OHyW to avoid fluctuations and prevent multiple iso-energetic conformations of hydroxywybutine side chain as compared to wybutine (yW).

Conformational preferences of modified nucleoside N(2)-methylguanosine (m(2)G) and its derivative N(2), N(2)-dimethylguanosine (m(2)(2)G) occur at 26th position (hinge region) in tRNA

Conformational preferences of the modified nucleosides N(2)-methylguanosine (m(2)G) and N(2), N(2)-dimethylguanosine (m(2)(2)G) have been studied theoretically by using quantum chemical perturbative configuration interaction with localized orbitals (PCILO) method. Automated complete geometry optimization using semiempirical quantum chemical RM1, along with ab initio molecular orbital Hartree-Fock (HF-SCF), and density functional theory (DFT) calculations has also been made to compare the salient features. Single-point energy calculation studies have been made on various models of m(2)G26:C/A/U44 and m(2)(2)G26:C/A/U44. The glycosyl torsion angle prefers "syn" (χ = 286°) conformation for m(2)G and m(2)(2)G molecules. These conformations are stabilized by N(3)-HC2' and N(3)-HC3' by replacing weak interaction between O5'-HC(8). The N(2)-methyl substituent of (m(2)G26) prefers "proximal" or s-trans conformation. It may also prefer "distal" or s-cis conformation that allows base pairing with A/U44 instead of C at the hinge region. Thus, N(2)-methyl group of m(2)G may have energetically two stable s-trans m(2)G:C/A/U or s-cis m(2)G:A/U rotamers. This could be because of free rotations around C-N bond. Similarly, N(2), N(2)-dimethyl substituent of (m(2)(2)G) prefers "distal" conformation that may allow base pairing with A/U instead of C at 44th position. Such orientations of m(2)G and m(2)(2)G could play an important role in base-stacking interactions at the hinge region of tRNA during protein biosynthesis process.

Iso-energetic multiple conformations of hypermodified nucleic acid base wybutine (yW) which occur at 37(th) position in anticodon loop of tRNA(Phe)

Conformational preferences of wybutine (yW) have been studied by quantum chemical semi-empirical Perturbative Configuration Interaction with Localized Orbitals (PCILO) and PM3 methods. Automated full geometry optimization by using RM1 along with ab-initio Hartree-Fock (HF-SCF) and Density Functional Theory (B3LYP/6-31G**) calculations have also been made to compare the salient features. Molecular dynamics (MD) simulation has been performed to see the solvation effect on wybutine side chain. The preferred conformations of wybutine side chain spreads away 'distal' from five membered imidazole moiety of tricyclic base. The intramolecular interactions provide stability to the preferred wybutine structure. The most stable and alternative stable structures obtained by PCILO and PM3 methods reveal that wybutine side chain may have multiple iso-energetic conformations. Molecular dynamics (MD) simulation study also confirms multiple conformations of wybutine side chain by showing regular periodical fluctuations over the 2 ns time period. These fluctuations occur when torsion angle α takes value ±90° and ±120° as observed in the most stable and alternative stable structures resulted by PCILO and PM3 methods. Such conformational behavior of wybutine may have certain implications on frameshifting to prevent extended Watson-Crick base pairing by maintaining proper codon-anticodon interactions during protein biosynthesis process.

Conformational preferences of hypermodified nucleoside lysidine (k2C) occurring at "wobble" position in anticodon loop of tRNA(Ile)

Conformational preferences of hypermodified nucleoside, 4-amino-2-(N(6)-lysino)-1-(beta-D-ribofuranosyl) pyrimidinium (Lysidine or 2-lysyl cytidine), usually designated as k(2)C, have been investigated theoretically by the quantum chemical perturbative configuration interaction with localized orbitals (PCILO) method. The zwitterionic, non-zwitterionic, neutral, and tautomeric forms have been studied. Automated geometry optimization using molecular mechanics force field (MMFF), semi-empirical quantum chemical PM3, and ab initio molecular orbital Hartree-Fock SCF quantum mechanical calculations have also been made to compare the salient features. The predicted most stable conformations of zwitterionic, non-zwitterionic, neutral, and tautomeric form are such that in each of these molecules the orientation of lysidine moiety (R) is trans to the N(1) of cytidine. The preferred base orientation is anti (chi = 3 degrees ) and the lysine substituent folds back toward the ribose ring. This results in hydrogen bonding between the carboxyl oxygen O(12a) of lysine moiety and the 2'-hydroxyl group of ribose sugar. In all these four forms of lysidine O(12a)...H-C(9) and O(12b)...H-N(11) interactions provide stability to respective stable conformers. Watson-Crick base pairing of lysidine with A is feasible only with the tautomeric form of usual anti oriented lysidine. This can help in recognition of AUA codon besides in avoiding misrecognition of AUG.

Decoding the genome: a modified view

Transfer RNA's role in decoding the genome is critical to the accuracy and efficiency of protein synthesis. Though modified nucleosides were identified in RNA 50 years ago, only recently has their importance to tRNA's ability to decode cognate and wobble codons become apparent. RNA modifications are ubiquitous. To date, some 100 different posttranslational modifications have been identified. Modifications of tRNA are the most extensively investigated; however, many other RNAs have modified nucleosides. The modifications that occur at the first, or wobble position, of tRNA's anticodon and those 3'-adjacent to the anticodon are of particular interest. The tRNAs most affected by individual and combinations of modifications respond to codons in mixed codon boxes where distinction of the third codon base is important for discriminating between the correct cognate or wobble codons and the incorrect near-cognate codons (e.g. AAA/G for lysine versus AAU/C asparagine). In contrast, other modifications expand wobble codon recognition, such as U*U base pairing, for tRNAs that respond to multiple codons of a 4-fold degenerate codon box (e.g. GUU/A/C/G for valine). Whether restricting codon recognition, expanding wobble, enabling translocation, or maintaining the messenger RNA, reading frame modifications appear to reduce anticodon loop dynamics to that accepted by the ribosome. Therefore, we suggest that anticodon stem and loop domain nucleoside modifications allow a limited number of tRNAs to accurately and efficiently decode the 61 amino acid codons by selectively restricting some anticodon-codon interactions and expanding others.