TRANS-ACTING FACTORS IN YEAST PRE-RIBOSOMAL-RNA AND PRE-SNORNA PROCESSING

The major intermediates in the pathway of pre-rRNA processing in yeast and other eukaryotes were originally identified by biochemical analys es. However, as a result of the analysis of the effects of mutations i n trans-acting factors, the yeast pre-rRNA processing pathway is now c haracterized in far more detail than that of other eukaryotes. These a nalyses have led to the identification of processing sites and interme diates that were either too close in size or too short lived to be det ected by biochemical analyses alone. In addition, it was generally unc lear whether pre-rRNA processing steps were endonucleolytic or exonucl eolytic; analyses of trans-acting factors is now revealing a complex m ixture of endonucleolytic and exonucleolytic processing steps. Many of the small nucleolar RNAs (snoRNAs) are excised from larger precursors . Analyses of trans-acting factors are also revealing details of pre-s noRNA processing in yeast. Interestingly, factors involved in pre-snoR NA processing turn out to be components that also function in pre-rRNA processing, suggesting a potential mechanism for the coregulation of rRNA and snoRNA synthesis. In general, very little is known about the regulation of pre-rRNA processing steps. The best candidate for a syst em regulating specific pre-rRNA processing reactions has recently been revealed by the analysis of a yeast pre-RNA methylase. Here we will r eview recent data on the trans-acting factors involved in yeast riboso me synthesis and discuss how these analyses have contributed to our cu rrent view of this complex process.