NUCLEAR MESSENGER-RNA ACCUMULATION CAUSES NUCLEOLAR FRAGMENTATION IN YEAST MTR2 MUTANT

We have identified a set of genes that affect mRNA transport (mtr) fro m the nucleus to the cytoplasm of Saccharomyces cerevisiae. One of the se genes, MTR2, has been cloned and shown to encode a novel 21-kDa nuc lear protein that is essential for vegetative growth. MTR2 shows limit ed homology to a protein implicated in plasmid DNA transfer in Escheri chia coli. PolyA(+)RNA accumulates within the nucleus of mtr2-1 in two to three foci at 37 degrees C. mRNA, tRNA, and rRNA synthesis continu e as do pre-mRNA splicing, tRNA processing, and rRNA export at 37 degr ees C. Under these conditions the polyA tail length increases, and pro tein synthesis is progressively inhibited. Nucleolar antigens also red istribute to two to three nuclear foci at 37 degrees C, and this redis tribution depends on ongoing transcription by RNA polymerase II. Surpr isingly, these foci coincide with the sites of polyA(+)RNA accumulatio n. Comparable colocalization and dependance on RNA polymerase II trans cription is seen for the mtr1-1 mutant. The disorganization of the nuc leolus thus depends on mRNA accumulation in these mutants. We discuss the possible functions of MTR2 and the yeast nucleolus in mRNA export.