= UUCG tetraloop
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A <UUCG tetraloop> is an <RNA tetraloop> whose four loop residues have <nucleotide sequence> U–U–C–G. It is one member of the <UNCG tetraloop> family, with U occupying the variable N position. The closing stem <base pair> lies outside those four residues.
The structurally studied sequence `5'-GGACUUCGGUCC-3'` illustrates the bookkeeping. Stem pairs 1–12,2–11,3–10 and 4–9 leave UUCG at positions 5–8, giving $2(4)+4=12$ <nucleotides>. The loop's first <uracil> and last <guanine> can themselves contact without changing its four-residue sequence definition.
Solution <NMR spectroscopy> determined interproton distances and scalar couplings for this hairpin. The resulting restrained structural model showed a loop G–U contact with syn-oriented guanosine, plus a cytosine–phosphate contact and organized <base stacking>. Mutating conserved loop residues reduced the measured stability.<Structure of an unusually stable RNA hairpin by Gabriele Varani and others>[Abstract, sequence, NMR restraints, syn-guanosine and mutation sensitivity.]{ref} The syn description concerns the <glycosidic torsion angle> of that nucleotide; it must not be confused with the cis/trans classification of relative <RNA base-pair geometry>.
The original paper describes only two residues as the remaining structural loop after its G–U contact extends the paired region. This is compatible with naming the four-residue sequence motif UUCG: a sequence-based tetraloop designation and a detailed contact-based loop count use different boundaries.<Structure of an unusually stable RNA hairpin by Gabriele Varani and others>[Abstract, G–U contact and two remaining loop nucleotides.]{ref}
A useful comparison keeps the stem and solution conditions fixed while changing one loop position. <RNA optical melting> tests the effect on a transition, while <NMR spectroscopy> tests the spatial contacts. Membership in a short sequence family does not fix a context-independent <RNA melting temperature> or prove that every larger <RNA> containing those letters adopts the same structure.
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