| The Small Nucle(ol)ar RNA Cap Trimethyltransferase Is Required for Ribosome Synthesis and Intact Nucleolar Morphology | |||||||||||||
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| Nucleolar morphogenesis is a poorly defined process. Here we report that the Saccharomyces cerevisiae nucleolar trimethyl guanosine synthase I (Tgs1p), which specifically selects the m7G cap structure of snRNAs and snoRNAs for m2,2,7G conversion, is required not only for efficient pre-mRNA splicing but also for pre-rRNA processing and small ribosomal subunit synthesis. Mutational analysis indicates that the requirement for Tgs1p in pre-mRNA splicing, but not its involvement in ribosome synthesis, is dependent upon its function in cap trimethylation. In addition, we report that cells lacking Tgs1p showed a striking and unexpected loss of nucleolar structural organization. Tgs1p is not a core component of the snoRNP proteins; however, in vitro, the protein interacts with the KKD/E domain present at the carboxyl-terminal ends of several snoRNP proteins. Strains expressing versions of the snoRNPs lacking the KKD/E domain were also defective for nucleolar morphology and showed a loss of nucleolar compaction. We propose that the transient and functional interactions of Tgs1p with the abundant snoRNPs, through presumed interactions with the KKD/E domain of the snoRNP proteins, contribute substantially to the coalescence of nucleolar components. This conclusion is compatible with a model of self-organization for nucleolar assembly. | |||||||||||||
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