A conditional yeast mutant deficient in mRNA transport from nucleus to cytoplasm

Transport of mRNA from nucleus to cytoplasm is critical for eukaryotic gene expression; however, the mechanism of export is unknown. Selection and screening procedures have therefore been used to obtain a family of temperature-sensitive conditional mutants of Saccharomyces cerevisiae that accumulate poly(A)⁺ RNA in the nucleus when incubated at 37°C, as judged by in situ hybridization. In one such mRNA transport mutant, mtr1-1, RNA synthesis continues, the export of poly(A)⁺ RNA is inhibited, intranuclear poly(A)⁺ is remarkably stable, and protein synthesis gradually stops. Thus, there is no tight coupling between RNA synthesis and export. The export lesion is reversible. Although mRNA export is clearly not a default option, neither inhibition of protein synthesis, inhibition of mRNA splicing, nor inhibition of poly(A)-binding protein function blocks export of the average poly(A)⁺, as judged by in situ hybridization. Further analysis of the family of mtr mutants should help map the path of RNA transport.