Localization of nuclear retained mRNAs in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, a common conditional phenotype associated with deletion or mutation of genes encoding mRNA export factors is the rapid accumulation of mRNAs in intranuclear foci, suggested to be near transcription sites. The nuclear RNA exosome has been implicated in retaining RNAs in these foci; on deletion of the exosome component Rrp6p, the RNA is released. To determine the exact nuclear location of retained as well as released mRNAs, we have used mRNA export mutant strains to analyze the spatial relationship between newly synthesized heat shock mRNA, the chromosomal site of transcription, and known S. cerevisiae nuclear structures such as the nucleolus and the nucleolar body. Our results show that retained SSA4 RNA localizes to an area in close proximity to the SSA4 locus. On deletion of Rrp6p and release from the genomic locus, heat shock mRNAs produced in the rat7-1 strain colocalize predominantly with nucleolar antigens. Bulk poly(A)(+) RNA, on the other hand, is localized primarily to the nuclear rim. Interestingly, the RNA binding nucleocytoplasmic shuttle protein Npl3p shows strong colocalization with bulk poly(A)(+) RNA, regardless of its nuclear location. Taken together, our data show that retention occurs close to the gene and indicate distinct nuclear fates of different mRNAs.