mRNA transport in yeast: time to reinvestigate the functions of the nucleolus.

Nucleocytoplasmic transport of mRNA is vital to gene expression and may prove to be key to its regulation. Genetic approaches in Saccharomyces cerevisiae have led to the identification of conditional mutants defective in mRNA transport. Mutations in approximately two dozen genes result in accumulation of transcripts, trapped at various sites in the nucleus, as detected by in situ hybridization. Phenotypic and molecular analyses of many of these mRNA transport mutants suggest that, in yeast, the function of the nucleus is not limited to the biogenesis of pre-ribosomes but may also be important for transport of poly(A)+ RNA. A similar function of the animal cell nucleolus is suggested by several observations.