Nuclear mRNA accumulation causes nucleolar fragmentation in yeast mtr2 mutant

We have identified a set of genes that affect mRNA transport (mtr) from the nucleus to the cytoplasm of Saccharomyces cerevisiae. One of these genes, MTR2, has been cloned and shown to encode a novel 21-kDa nuclear protein that is essential for vegetative growth. MTR2 shows limited homology to a protein implicated in plasmid DNA transfer in Escherichia coli. PolyA⁺RNA accumulates within the nucleus of mtr2-1 in two to three foci at 37°C, mRNA, tRNA, and rRNA synthesis continue as do pre-mRNA splicing, tRNA processing, and rRNA export at 37°C. Under these conditions the polyA tail length increases, and protein synthesis is progressively inhibited. Nucleolar antigens also redistribute to two to three nuclear foci at 37°C, and this redistribution depends on ongoing transcription by RNA polymerase II. Surprisingly, these foci coincide with the sites of polyA⁺RNA accumulation. Comparable colocalization and dependence on RNA polymerase II transcription is seen for the mtr1-1 mutant. The disorganization of the nucleolus thus depends on mRNA accumulation in these mutants. We discuss the possible functions of MTR2 and the yeast nucleolus in mRNA export.