A diverse set of nuclear RNAs transfer between nuclei of yeast heterokaryons

Small nuclear RNAs and small nucleolar RNAs function in the nucleus of eukaryotic cells during pre-mRNA splicing and ribosomal RNA processing, respectively. In metazoan cells, the small nuclear RNAs shuttle between the nucleus and the cytoplasm during ribonucleoprotein particle assembly. Nuclear export of these small RNAs in yeast, however, has not been demonstrated. Therefore, we have attempted to visualize internuclear RNA movements by in situ hybridization in heterokaryon yeast cells. Using the kar1 Δ15 mutation to block karyogamy, we mated two strains, each expressing a unique allele of U1 snRNA. In these heterokaryons, we observed a time-dependent transfer of U1 snRNA from one nucleus to the other. This transfer was reduced two-fold by the addition of the Crm1p-inhibitor leptomycin B. Interestingly, however, we observed identical transfer of the U2 and U6 snRNAs and SNR4, SNR8, SNR9 and SNR11 snoRNAs. Remarkably, when the U2, U6 or SNR4 RNAs were observed in the same heterokaryon as the U1 snRNA, both RNAs always transferred simultaneously. These data suggest a global leaking or transport of material between nuclei of yeast heterokaryons. Our results suggest that caution must be taken when testing nuclear envelope shuttling in yeast heterokaryons.