These ancient, complex nanomachines consist of two ribonucleoprotein subunits. RIBOSOMES are the cellular machines that translate the genetic code in mRNA and catalyze protein synthesis in all organisms. Studies of yeast ribosome biogenesis provide useful models for ribosomopathies, diseases in humans that result from failure to properly assemble ribosomes. Elaborate mechanisms exist to monitor the formation of correct structural and functional neighborhoods within ribosomes and to destroy preribosomes that fail to assemble properly. Following export of preribosomes from the nucleus to the cytoplasm, they undergo final stages of maturation before entering the pool of functioning ribosomes. More than 200 assembly factors and 76 small nucleolar RNAs transiently associate with assembling ribosomes, to enable their accurate and efficient construction. Assembly begins with transcription of ribosomal RNA in the nucleolus, where the RNA then undergoes complex pathways of folding, coupled with nucleotide modification, removal of spacer sequences, and binding to ribosomal proteins.

During the past 25 years, studies in yeast have led the way to understanding how these molecules are assembled into ribosomes in vivo. In yeast these complex particles contain four RNAs (>5400 nucleotides) and 79 different proteins. Ribosomes are highly conserved ribonucleoprotein nanomachines that translate information in the genome to create the proteome in all cells.

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