During the nucleolar maturation of eukaryotic ribosomal RNAs, many selected uridines are converted into pseudouridine by a thus far undefined mechanism. The nucleolus contains a large number of small RNAs (snoRNAs) that share two conserved sequence elements, box H and ACA. In this study, we demonstrate that site-specific pseudouridylation of rRNAs relies on short ribosomal signal sequences that are complementary to sequences in box H/ACA snoRNAs. Genetic depletion and reconstitution studies on yeast snR5 and snR36 snoRNAs demonstrate that box H/ACA snoRNAs function as guide RNAs in rRNA pseudouridylation. These results define a novel function for snoRNAs and further reinforce the idea that base pairing is the most common way to obtain specific substrate-"enzyme" interactions during rRNA maturation.

Eukaryotic cells contain a large number of small nucleolar RNAs (snoRNAs).A major family of snoRNAs features a consensus ACA motif positioned 3 nucleotides from the 3' end of the RNA. In this study we have characterized nine novel human ACA snoRNAs (U64-U72). Structural probing of U64 RNA followed by systematic computer modeling of all known box ACA snoRNAs revealed that this class of snoRNAs is defined by a phylogenetically conserved secondary structure. The ACA snoRNAs fold into two hairpin structures connected by a single-stranded hinge region and followed by a short 3' tail. In eukaryotic cells, maturation of rRNAs takes place in the nucleolus. The 18S, 5.8S, and 25/28S rRNAs are syn thesized as a large precursor RNA (pre-rRNA) that car ries long external and internal spacer sequences. Shortly after transcription, many nucleotides in the coding re gions of pre-rRNA are methylated at the 2'-0-hydroxyl position and numerous U residues are converted into pseudouridines (Maden 1990;Eichler and Craig 1995). The covalently modified pre-rRNA is than processed into mature rRNAs through a series of endo-and exonucleolytic cleavages (Eichler and Craig 1995; Venema and Tollervey 1995;Sollner-Webb et al. 1996).The nucleolus contains a large number of snoRNAs. More than 80 distinct small nucleolar RNA (snoRNA) sequences have been identified in vertebrate and yeast

Trophic endosymbiosis between anthozoans and photosynthetic dinoflagellates forms the key foundation of reef ecosystems. Dysfunction and collapse of symbiosis lead to bleaching (symbiont expulsion), which is responsible for the severe worldwide decline of coral reefs. Molecular signals are central to the stability of this partnership and are therefore closely related to coral health. To decipher inter-partner signaling, we developed genomic resources (cDNA library and microarrays) from the symbiotic sea anemone Anemonia viridis. Here we describe differential expression between symbiotic (also called zooxanthellate anemones) or aposymbiotic (also called bleached) A. viridis specimens, using microarray hybridizations and qPCR experiments. We mapped, for the first time, transcript abundance separately in the epidermal cell layer and the gastrodermal cells that host photosynthetic symbionts. Transcriptomic profiles showed large inter-individual variability, indicating that aposymbiosis could be induced by different pathways. We defined a restricted subset of 39 common genes that are characteristic of the symbiotic or aposymbiotic states. We demonstrated that transcription of many genes belonging to this set is specifically enhanced in the symbiotic cells (gastroderm). A model is proposed where the aposymbiotic and therefore heterotrophic state triggers vesicular trafficking, whereas the symbiotic and therefore autotrophic state favors metabolic exchanges between host and symbiont. Several genetic pathways were investigated in more detail: i) a key vitamin K–dependant process involved in the dinoflagellate-cnidarian recognition; ii) two cnidarian tissue-specific carbonic anhydrases involved in the carbon transfer from the environment to the intracellular symbionts; iii) host collagen synthesis, mostly supported by the symbiotic tissue. Further, we identified specific gene duplications and showed that the cnidarian-specific isoform was also up-regulated both in the symbiotic state and in the gastroderm. Our results thus offer new insight into the inter-partner signaling required for the physiological mechanisms of the symbiosis that is crucial for coral health.