Ribosome Assembly I

Ribosome assembly takes place in the nucleolus. The assembly of the 40S subunit (exemplified in the illustration) starts as the giant primary transcript is being produced. During this period initial folding and snoRNA-dependent cleavage of the primary transcript take place. Other snoRNAs direct modification of specific nucleotides in the rRNA. Primary rRNA binding ribosomal proteins associate to the rRNA and the folding of the rRNA proceeds. The next set of ribosomal proteins, i.e. proteins which do not bind directly to rRNA, join the rRNA-protein complex thereby forming the 40S ribosomal subunit. For production of the 60S subunit the assembly process also has to be coordinated with the production of 5S rRNA. The whole ribosome synthesis and assembly process is an intricate interplay between the nucleolus, the nucleus and the cytoplasm.