Viewing the ribosome and visiting the nucleolus at Lake Tahoe

A meeting “Ribosome Biogenesis and Nucleolar Function” was held at Lake Tahoe, California,August 17–21, 2000, ably organized by Lasse Lindahl and Janice Zengel (University of Maryland, Baltimore County), together with Maurille Fournier (University of Massachusetts),Craig Pikaard (Washington University), and John Woolford (Carnegie-Mellon University)+The meeting was noteworthy in several respects+ First, it occurred within only a few days (on either side) of the publication of breathtaking 2+4–3+3 A structures of the large and small ribosomal subunits+ Second, it addressed how this machine is built in the cell to a greater extent than most previous ribosome meetings+ Third, the meeting congratulated Masayasu Nomura for his important, and still continuing, contributions to the ribosome field+ After a memorial tribute in the opening session to the late Rudi Planta, the meeting’s organization unfolded with Aristotelian logic, starting with rRNA gene expression, in which welcome biochemical and genetic progress on the ribosomal gene transcription machinery was presented+ With the possible exception of pol I itself, all the rDNA transcription components are now pure proteins and we await the next (crystallographic) steps+ A longstanding issue has been whether nucleoli are simply the cytological manifestation of rDNA transcription and nascent ribosome assembly or are first assembled by other factors+Human acrocentric chromosomes display a remarkable propensity to associate with ribosomal RNA genes—the nucleolar organizers, which can end up constituting 25–33% of the entire short arm of chromosomes 13, 14, 15, 21, and 22+ These arms expectedly nucleate prenucleoli in telophase, which then fuse to form the larger nucleoli that typify the interphase landscape+ Surprisingly, this essential role of the rDNA in nucleolar assembly does not require its transcriptional activity, as revealed by a clever exploitation of species-specific rDNA transcription in mouse/human cell hybrids (B+ McStay, University of Dundee)+ Thus, it appears that the initial signal for assembling a nucleolus is something more primordial that rDNA transcription itself+ A look into intriguing relationships between the nucleolus and rDNA expression on the one hand and broader aspects of cell regulation on the other was offered by Masayasu Nomura (University of California, Irvine)+ The 35S rRNA genes on chromosome XII in Saccharomyces cerevisiae have an enhancer at the end of each module in the repeated array, initially identified by Jonathan Warner’s lab and further characterized by Shirleen Roeder and colleagues in connection with their work on HOT1-dependent stimulation of recombination+ This enhancer’s role was defined using ectopic reporter genes linked to the pol I promoter+ Yet, Nomura reported that deletion strains of this putative enhancer grow at wild-type rates and display normal rRNA synthesis+ In addition, he showed that another mutation, fob1, abolishes enhancer-dependent transcription at an ectopic pol I promoter in a HOT1 system, without any effect on chromosomal rDNA transcription+ The FOB1 gene encodes a hot-spot related protein, which plays an essential role in recombination within the rDNA repeats+ Nomura’s results imply that the FOB1 gene product together with the enhancer plays a role in recruiting pol I and related transcription machinery to ectopic rDNA in the previous studies+ He suggests that the FOB1 gene product stimulates an interaction between rDNA repeats through the enhancer, explaining its role in recombination as well as its requirement for transcription from ectopic pol I promoters+ These results emphasize that the nucleolus Reprint request to: Thoru Pederson, Department of Biochemistry and Molecular Biology, University of Massachusetts Medical School, Worcester, Massachusetts 01605, USA; e-mail: thoru+pederson@ umassmed+edu+ RNA (2001), 7:1–4+ Cambridge University Press+ Printed in the USA+ Copyright © 2001 RNA Society+