Small nucleolar RNAs and pre-rRNA processing in plants.

Messenger RNA (mRNA) translation and thereby gene expression depend on the cell’s ability to produce vast numbers of ribosomes, the major component of the translational machinery. The complexity inherent in the production of ribosomal RNAs (rRNAs), the key RNA components of the ribosome, is interesting in itself. Add to this the complex nature of the production and regulation of the myriad other components involved in ribosome biogenesis, and this topic becomes fascinating. Indeed, there has been a recent resurgence of interest in the production of rRNA, the most abundant RNA in the cell, and in the cell biology of the nuclear domain in which ribosome biogenesis occurs—the nucleolus. This interest has been largely stimulated by the discovery of large families of small nucleolar RNAs (snoRNAs) and the analysis of their functions in rRNA production and ribosome biogenesis. Recent advances in resolving the structure of the nucleolus and in the number of protein and RNA probes that are available have also made it possible to relate subcellular structure to molecular events. This is particularly true in plants, in which the superior resolution afforded by the plant nucleolus puts plant science in this area at the forefront of nucleolar structure–function investigations. In this review, we describe recent studies of rRNA processing, concentrating mainly on the role of snoRNAs. We also compare the different modes of snoRNA production in different eukaryotic kingdoms, highlight the novel organization of snoRNA genes in plants, and present a model that synthesizes our current understanding of the organization of rRNA processing events in the nucleolus.