Mouse and human Ring1B 5′ UTRs show a striking structural resemblance.

<p>Secondary structure prediction of the Ring1B IRES of human and mouse origin, showing a striking structure similarity (A). The first nucleotide is indicated with a black circle around it, the last triplet is the translational start. Loop numbers are indicated with filled black circles. In mutant IRES*1 and IRES*2 the indicated cytosines are replaced by adenines, creating mismatches. In mutant IRES*3 the indicated uracil is replaced by a cytosine, creating a matching nucleotide and therefore stabilizing the structure. Mutant IRES*4 lacks the entire second loop. Luciferase assays show the remaining activity of the mutant IRESes and the stabilized activity in case of the matching mutation (B). The same construct as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0002322#pone-0002322-g003" target="_blank">figure 3a</a>, construct number 1, was used, but with the mutations introduced. The way of interpreting the graph is identical to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0002322#pone-0002322-g003" target="_blank">figure 3b–f</a>. The mismatch point mutations almost completely abolish the IRES activity, whereas IRES*3 is, as expected, not affected by its stabilizing point mutation. Surprisingly, the IRES lacking the entire second loop still has some residual activity, indicating the relative importance of the remaining structure.