In the yeast Saccharomyces cerevisiae, as in in most other eukaryotic cells, the 18S, 5.8S and 25S rRNAs are transcribed by RNA polymerase I as a single precursor molecule containing both internal (ITS1 and ITS2) and external (5'ETS and 3'ETS) transcribed spacers, which are subsequently removed in an ordered series of endo- and exonucleolytic cleavages. While the position of these cleavages and the order in which they occur is now well-established our knowledge of the molecular mechanisms underlying rRNA maturation and the manner in which pre-rRNA processing is linked to assembly with the ribosomal proteins is still very limited. Over the past years it has become clear, however, that pre-rRNA maturation and assembly are highly complex processes both guided by structural elements within the precursor (cis-acting elements) and requiring the participation of numerous trans-acting (non-ribosomal) factors, whose precise role remains largely to be established. Our current interest focuses on several of these trans-acting factors. In particular we are studying Rrp5p, one of only two proteins known to be required for biogenesis of both the large and small ribosomal subunit; Rio2p, a novel type of protein kinase, that is essential for the final, cytoplasmic step in small subunit maturation and Rrp9p, a component of the U3 snoRNP particle that initiates the formation of a large processing complex on the primary pre-rRNA transcript. In this research we employ a wide array of genetic (e.g. suppressor analysis, synthetic lethality ), molecular-biological and biochemical techniques.