The genetic material of eukaryotes is surrounded by a nuclear envelope, which acts as a barrier to the free exchange of proteins and nucleic acids between the nucleoplasm and cytoplasm. Although the resulting physical separation of transcription and translation provides cells with a level of control of gene expression not available to prokaryotes, it also demands the presence of a transport system to selectively move macromolecules between the two compartments. Such a system must be capable of discriminating between a wide variety of structurally and functionally distinct molecules and may also respond to differing growth and environmental conditions.
The biosynthesis and transport of ribosomes in a rapidly growing cell (such as the yeast Saccharomyces cerevisiae) presents a dramatic paradigm for the magnitude of the problems faced by the nucleocytoplasmic transport system. Each mRNA encoding a ribosomal protein must...
