A surfeit of factors: why is ribosome assembly so much more complicated in eukaryotes than bacteria?

Recent years have seen a dramatic increase in the number of ribosome synthesis factors identified in the yeast Saccharomyces cerevisiae. Most of these are not predicted to directly catalyze either RNA processing or modification, and they are therefore predicted to function in some sense as assembly factors, promoting the assembly and/or disassembly of the processing and modification machinery, binding of the ribosomal proteins and correct folding of the pre-rRNAs and rRNAs. In contrast, ribosome synthesis in E. coli, which has also been extensively analyzed, appears to involve a very small number of potential assembly factors. Here we will consider the differences between eukaryotic and bacterial ribosome synthesis that may underlie this distinction.