The Neurospora VS ribozyme recognizes and cleaves a substrate RNA that contains a GC-rich stem loop. In contrast to most RNA secondary structures that are stable during tertiary or quaternary folding, this substrate undergoes extensive ribozyme-induced rearrangement in the presence of magnesium in which the base pairings of at least seven of the ten nucleotides in the stem are changed. This conformational switch is essential for catalytic activity with the wild-type substrate and creates a metal-binding secondary structure motif near the cleavage site. Base pair rearrangement is accompanied by bulging a cytosine from the middle of the stem, indicating that ribozymes may perform base flipping, an activity previously observed only with protein enzymes that modify DNA.

Kissing interactions in RNA are formed when bases between two hairpin loops pair. Intra-and intermolecular kissing interactions are important in forming the tertiary or quaternary structure of many RNAs. Self-cleavage of the wild-type Varkud satellite (VS) ribozyme requires a kissing interaction between the hairpin loops of stemloops I and V. In addition, self-cleavage requires a rearrangement of several base pairs at the base of stem I. We show that the kissing interaction is necessary for the secondary structure rearrangement of wild-type stem-loop I. Surprisingly, isolated stem-loop V in the absence of the rest of the ribozyme is sufficient to rearrange the secondary structure of isolated stem-loop I. In contrast to kissing interactions in other RNAs that are either confined to the loops or culminate in an extended intermolecular duplex, the VS kissing interaction causes changes in intramolecular base pairs within the target stem-loop.R NA kissing interactions, also called loop-loop pseudoknots, occur when the unpaired nucleotides in one hairpin loop base pair with the unpaired nucleotides in another hairpin loop (1). When the hairpin loops are located on separate RNA molecules, their intermolecular interaction is called a kissing complex. These interactions generally form between stem-loops containing extensive complementarity; however, stable complexes have been observed containing only two intermolecular Watson-Crick base pairs (2).Intramolecular kissing interactions are observed in the native structures of a variety of RNAs including Varkud satellite (VS) RNA, tRNA, and group I introns (3-6). These kissing interactions contribute to the assembly and stabilization of their respective RNA structures by joining and orienting helices. Kissing interactions may stabilize both native and nonnative interactions during tertiary folding, which can affect the rate at which the native structure is formed (7). Kissing interactions often form distorted structures that can serve as recognition sites for proteins, RNAs, metal ions, or other ligands (8-10). As a result, kissing interactions contribute to the stability of an RNA structure by affecting both global and local RNA interactions.The transient formation of an intermolecular kissing complex is required for RNA dimerization during the life cycle of retroviruses (11-15), and for the formation of some antisensetarget complexes (16,17). Kissing complexes in which the loop nucleotides are complementary can form stable dimers that contain intermolecular base pairs between the loop nucleotides only. Other stem-loops with more extensive complementarity sometimes form unstable kissing complexes, which are quickly remodeled into stable duplex or cruciform isoforms (18)(19)(20). Secondary structure remodeling involves breaking intramolecular base pairs in each hairpin and using the bases to form intermolecular helices.The VS ribozyme contains a kissing interaction that is required for self-cleavage of the wild-type RNA in vitro (6). This interaction involves Watson-Crick bas...

One X chromosome, selected at random, is silenced in each female mammalian cell. Xist encodes a noncoding RNA that influences the probability that the cis-linked X chromosome will be silenced. We found that the A-repeat, a highly conserved element within Xist, is required for the accumulation of spliced Xist RNA. In addition, the A-repeat is necessary for X-inactivation to occur randomly. In combination, our data suggest that normal Xist RNA processing is important in the regulation of random X-inactivation. We propose that modulation of Xist RNA processing may be part of the stochastic process that determines which X chromosome will be inactivated.

Substrate cleavage by the Neurospora Varkud satellite (VS) ribozyme involves a structural change in the stem-loop I substrate from an inactive to an active conformation. We have determined the NMR solution structure of a mutant stem-loop I that mimics the active conformation of the cleavage site internal loop. This structure shares many similarities, but also significant differences, with the previously determined structures of the inactive internal loop. The active internal loop displays different base-pairing interactions and forms a novel RNA fold composed exclusively of sheared G-A base pairs. From chemical-shift mapping we identified two Mg 2؉ binding sites in the active internal loop. One of the Mg 2؉ binding sites forms in the active but not the inactive conformation of the internal loop and is likely important for catalysis. Using the structure comparison program MC-SEARCH, we identified the active internal loop fold in other RNA structures. In Thermus thermophilus 16S rRNA, this RNA fold is directly involved in a long-range tertiary interaction. An analogous tertiary interaction may form between the active internal loop of the substrate and the catalytic domain of the VS ribozyme. The combination of NMR and bioinformatic approaches presented here has identified a novel RNA fold and provides insights into the structural basis of catalytic function in the Neurospora VS ribozyme. RNA molecules play essential roles in many cellular processes. These include the enzymatic activity of ribozymes that are required for protein synthesis and certain RNA processing reactions (1). NMR and x-ray crystallographic studies have provided some insights into the relationship between RNA structure and catalysis; however, interpretation of structurefunction relationships, even in the well studied hammerhead ribozyme (2), continues to be challenging (1). It has also been difficult to make any generalizations about the role of RNA structure in catalysis, in part because of the small number of known ribozymes and the limited amount of structural information available. We are studying the Neurospora Varkud satellite (VS) ribozyme to provide information about the role of tertiary structure and conformational changes in RNA catalysis.The Neurospora VS ribozyme originates from an abundant RNA satellite of 881 nt found in the mitochondria of the Varkud-1c strain of Neurospora (3). Fragments of Ϸ120-180 nt derived from this natural RNA sequence undergo self-cleavage at a specific phosphodiester bond to produce 5Ј-OH and 2Ј,3Ј-cyclic phosphate termini (Fig. 1a) (3-5). Although these products are characteristic of other small ribozymes, the VS ribozyme possesses unique primary (4), secondary (6), and tertiary structures (7-9). The secondary structure of the self-cleaving VS ribozyme is characterized by six helical domains (Fig. 1a); stem-loop I forms the substrate domain and stem-loops II-VI comprise the catalytic domain (6). When these two domains are synthesized separately, the catalytic domain can perform the same cleavage reaction,...