Increased protection from reactive oxygen species (ROS) is believed to increase life span. However, it has not been clearly demonstrated that endogenous ROS production actually limits normal life span. We have identified a mutation in the Caenorhabditis elegans iron sulfur protein (isp-1) of mitochondrial complex III, which results in low oxygen consumption, decreased sensitivity to ROS, and increased life span. Furthermore, combining isp-1(qm150) with a mutation (daf-2) that increases resistance to ROS does not result in any significant further increase in adult life span. These findings indicate that both isp-1 and daf-2 mutations increase life span by lowering oxidative stress and result in the maximum life span increase that can be produced in this way.

We have investigated the secondary structure of peach latent mosaic viroid (PLMVd) in solution, and we present here the first description of the structure of a branched viroid in solution. Different PLMVd transcripts of plus polarity were produced by using the circularly permuted RNA method and the exploitation of RNA internal secondary structure to position the 5 and 3 termini and studied by nuclease mapping and binding shift assays using DNA and RNA oligonucleotides. We show that PLMVd folds into a complex, branched secondary structure. In general, this structure is similar to that reported previously, which was based on sequence comparison and computer modelling. The structural microheterogeneity is apparently limited to only some small domains. More importantly, this structure includes a novel pseudoknot that is conserved in all PLMVd isolates and seems to allow folding into a very compact form. This pseudoknot is also found in chrysanthemum chlorotic mottle viroid, suggesting that it is a unique feature of the viroid members of the PLMVd subgroup.Viroids are small (ϳ300 nucleotides), single-stranded, circular RNAs that infect higher plants, causing significant losses in the agricultural industry (see references 7 and 13 for reviews). Viroids have been classified in two groups (groups A and B) based primarily on whether or not they possess five typical structural domains found in the group B viroids (7). Further division among the group B members depends on the sequence and length of the conserved central region. Viroids that do not possess any kind of sequence or structural similarity with the group B viroids have been classified as belonging to group A. The viroids from this group possess self-cleaving hammerhead motifs that are crucial for their replication via a rolling circle mechanism.The group A viroids include the avocado sunblotch viroid (ASBVd), the peach latent mosaic viroid (PLMVd), and the chrysanthemum chlorotic mottle viroid (CChMVd) (10). Both PLMVd and CChMVd have been proposed to adopt branched secondary structures (Fig. 1A) instead of the rod-like ones proposed for most viroids, including ASBVd (10). The unusual conformations of PLMVd and CChMVd are supported by their insolubility in 2 M lithium chloride, whereas ASBVd and a number of non-self-cleaving viroids (i.e., the group B viroids) are soluble in this high salt solution (10). In general, secondary structures of viroids are predicted using computer software and are useful for the formulation of hypotheses on the structure-function relationships of these RNA molecules (4). Characterization of biological structures in vitro as well as in vivo is obviously more accurate for elucidating the structure-function relationship. The only secondary structure of a viroid that has been extensively studied in solution is that of the potato spindle tuber viroid (8). This group B species, which was shown to adopt a rod-like shape in solution, is responsible for most of our knowledge of the biology of viroids.In order to determine the secondary struct...

Hammerhead self-cleavage of dimeric, monomeric, truncated and mutated transcripts derived from both polarities of the peach latent mosaic viroid (PLMVd) were characterized. In contrast to some results previously published for a very close sequence variant (see ref. 1), these RNAs exhibit a virtually identical selfcleavage during transcription and after purification. By self-cleavage of dimeric transcripts with normal and mutated hammerhead domains and by complementation experiments, we show that the cleavage reactions involve only single hammerhead structures. This observation contrasts with the case of avocado sunblotch viroid (ASBVd), the other self-cleaving viroid, whose mechanism involves mostly double hammerhead structures, whereas single hammerhead cleavage is associated with viroid-like plant satellite RNAs. The difference in stability between the native secondary structures adopted by viroids and the autocatalytic structures, including the hammerhead motif, governs the efficiency of the self-cleavage reaction. The transition between these conformers is the limiting step in catalysis and is related exclusively to the left arm region of PLMVd secondary structure, which includes the hammerhead sequences. Most of the mutations between the variant we used and the sequence variant previously published are located in this left arm region, which may explain to a great extent the differences in their cleavage efficiency. No interactions with long-range sequences contributing to the autocatalytic tertiary structure were revealed in these experiments.

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We characterized the peach latent mosaic viroid (PLMVd) replication intermediates that accumulate in infected peach leaves and determined the tissue and subcellular localization of the RNA species. Using in situ hybridization, we showed that PLMVd strands of both plus and minus polarities concentrate in the cells forming the palisade parenchyma. At the cellular level, PLMVd was found to accumulate predominantly in chloroplasts. Northern blot analyses demonstrated that PLMVd replicates via a symmetric mode involving the accumulation of both circular and linear monomeric strands of both polarities. No multimeric conformer was detected, indicating that both strands self-cleave efficiently via their hammerhead sequences. Dot blot hybridizations revealed that PLMVd strands of both polarities accumulate equally but that the relative concentrations vary by more than 50-fold between peach cultivars. Taken together these results establish two hallmarks for the classification of viroids. Group A viroids (e.g., PLMVd), which possess hammerhead structures, replicate in the chloroplasts via the symmetric mode. By contrast, group B viroids, which share a conserved central region, replicate in the nucleus via an asymmetric mechanism. This is an important difference between self-cleaving and non-self-cleaving viroids, and the implications for the evolutionary origin and replication are discussed.