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Basepairing2

This short, linear chain of ribonucleotides folds back on itself to create the three base pairs of EteRNA: GU, AU, and GC.

Base pairs are the building blocks of stucture in EteRNA. They form when complementary bases associate with one another. Three pairs are possible in EteRNA:

Base pairs with unusual geometries and the interactions between mismatched bases are not explicitly depicted in EteRNA.

IntroductionEdit

Base pairs can be considered the "glue" that holds together a folded RNA. Breaking a base pair that has already formed requires the addition of energy to the RNA.

FormationEdit

Base pairing results from a combination of hydrogen bonding and hydrophobic stacking interactions. These interactions are only favorable with certain combinations of bases in specific orientations.

StrengthEdit

Base pairs vary in strength. The GC pair is stronger than AU or GU pairs due to the presence of an additional hydrogen bond and stronger stacking interactions. Additionally, the energy of a base pair can be altered by exchanging the positions of two paired bases. A GC pair with guanine in the 5' direction of cytosine will not make the same energy contribution as a CG pair with guanine in the 3' direction of cytosine.

Watson-Crick Base PairsEdit

Wobblevswc
The AU pair and GC pair are known as Watson-Crick base pairs. These are the most commonly encountered type of base pairs in double-stranded regions of RNA. The geometry of these two pairs in double-stranded RNA is nearly identical.

Watson-Crick Pairs in EteRNAEdit

The GC pair is stronger than the AU pair by up to 2 kcal/mol, and it is more stabilizing when it appears as a closing base pair.

Wobble PairsEdit

Main Article: Translation

Wobble pairs are non-Watson-Crick pairs involving bases at the final "wobble" position of the codon-anticodon pair. The ability to form these pairs allows a single tRNA to recognize multiple codons in mRNA. One of the first wobble pairs to be recognized is the GU pair. Other common pairings that occur in this position involve the nonstandard purine base inosine and cytosine, adenine, or uracil.

The GU pair is not restricted to the wobble position in the codon/anticodon pair. It is common in natural RNAs, often appearing in regions of the molecule where protein-RNA interactions occur.

The geometry of the GU pair differs from that of the AU and GC pairs. The distortion of the double-helix disrupts stacking interactions in the 5' direction of guanine. Closing GU pairs in nature are usually oriented in a way that places the first unpaired base of the loop 5' of guanine.
Wobble

Energy of adjacent GU Pairs in EteRNA.

Wobble Pairs in EteRNAEdit

In EteRNA, only the GU wobble pair is represented. When flanked by AU or GC pairs, the GU pair behaves similar to AU pair. When flanked by other GU pairs, the GU pair is very weak or destabilizing.

Other Noncanonical Base PairsEdit

Provided the right environment and orientation, any combination of the four common RNA bases can form hydrogen bonds. Additionally, three or more bases may interact to form triplex and quadruplex structures. These noncanonical base pairs can be seen in the atomic-level structures of many RNAs, but energy parameters for these pairs do not exist and they cannot be predicted from sequence.

Noncanonical Base Pairs in EteRNAEdit

Noncanonical pairs are not displayed in EteRNA; however, the energy bonus from boosting reflects the formation of certain noncanonical pairs in loops.

Base Pairs at the Atomic LevelEdit

Further Information: Jena Library

When atomic-level structural information is available, base pairs can be further classified according to the orientation of the interacting nucleotides. Base pairs are frequently described according to the type of pair, the interacting base edges, and orientation of the glycosidic bond.

Base Edge
Base Edges
Edit

In Watson-Crick base pairs, hydrogen bonding between bases involves a specific region of each nucleotide. Nucleotides contain many hydrogen bond donors and acceptors outside of this region that can also participate in base-pairing. Hydrogen bonding between two bases can involve any combination of three general regions, or edges.

Glycosidic Bond OrientationEdit

A glycosidic bond joins the sugar, ribose, and the base in a nucleotide. The glycosidic bonds of the two bases in a base pair can be oriented in the same (cis) or opposing (trans) directions.

Tautomeric FormEdit

Tautomerization alters the arrangement of hydrogen bond donors and acceptors in a base and can allow the formation of additional hydrogen bonds.

Water MoleculesEdit

In certain base pairs, particularly the U-U pair, pairing ccurs through a water molecule located between the two bases. Both bases form hydrogen bonds to the same water molecule.

Common NamesEdit

Certain noncanonical pairs have common names. For example, a "sheared GA pair" involves hydrogen bonding between the Hoogsteen and Sugar edges of guanine and adenine with a trans- orientation of the glycosidic bond.

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