A PERINUCLEOLAR COMPARTMENT CONTAINS SEVERAL RNA-POLYMERASE-III TRANSCRIPTS AS WELL AS THE POLYPYRIMIDINE TRACT-BINDING PROTEIN, HNRNP-I

We have investigated the subcellular organization of the four human Y RNAs. These RNAs, which are transcribed by RNA polymerase III, are usu ally found complexed with the Po autoantigen, a 60-kD protein. We desi gned 2'-OME oligoribonucleotides that were complementary to accessible single-stranded regions of Y RNAs within Ro RNPs and used them in flu orescence in situ hybridization. Although all four Y RNAs were primari ly cytoplasmic, oligonucleotides directed against three of the RNAs hy bridized to discrete structures near the nucleolar rim. We have termed these structures ''perinucleolar compartments'' (PNCs). Double labeli ng experiments with appropriate antisera revealed that PNCs are distin ct from coiled bodies and fibrillar centers. Co-hybridization with a g enomic DNA clone spanning the human Y1 and Y3 genes showed that PNCs a re not stably associated with the transcription site for these Y RNAs. Although 5S rDNA was often located near the nucleolar periphery, PNCs are not associated with 5S gene loci. Two additional pol III transcri pts, the RNA components of RNase P and RNase MRP, did colocalize withi n PNCs. Most interestingly, the polypyrimidine tract-binding protein h nRNP I/PTB was also concentrated in this compartment, Possible roles f or this novel nuclear subdomain in macromolecular assembly and/or nucl eocytoplasmic shuttling of these five pol III transcripts, along with hnRNP I/PTB, are discussed.