THE PERINUCLEOLAR COMPARTMENT AND TRANSCRIPTION

The perinucleolar compartment (PNC) is a unique nuclear structure loca lized at the periphery of the nucleolus. Several small RNAs transcribe d by RNA polymerase III and two hnRNP proteins have been localized in the PNC (Ghetti, A., S. Pinol-Roma, W.M. Michael, C. Morandi, and G. D reyfuss. 1992. Nucleic Acids Res. 20:3671-3678; Matera, A.G., M.R. Fre y, K. Margelot, and S.L. Wolin. 1995. J. Cell Biol. 129:1181-1193; Tim chenko, L.T., J.W. Miller, N.A. Timchenko, D.R. DeVore, K.V. Datar, L. Lin, R. Roberts, C.T. Caskey, and M.S. Swanson. 1996. Nucleic Acids l ies. 24: 4407-4414; Huang, S., T. Deerinck, M.H. Ellisman, and D.L. Sp ector. 1997. J. Cell Biol. 137:965-974). In this report, we show that the PNC incorporates Br-UTP and FITC-conjugated CTP within 5 min of pu lse labeling. Selective inhibition of RNA polymerase I does not apprec iably affect the nucleotide incorporation in the PNC. Inhibition of al l RNA polymerases by actinomycin D blocks the incorporation completely , suggesting that Br-UTP incorporation in the PNC is due to transcript ion by RNA polymerases II and/or III. Treatment of cells with an RNA p olymerase II and III inhibitor induces a significant reorganization of the PNC. In addition, double labeling experiments showed that poly(A) RNA and some of the factors required for pre-mRNA processing were loc alized in the PNC in addition to being distributed in their previously characterized nucleoplasmic domains. Fluorescence recovery after phot obleaching (FRAP) analysis revealed a rapid turnover of polypyrimidine tract binding protein within the PNC, demonstrating the dynamic natur e of the structure. Together, these findings suggest that the PNC is a functional compartment involved in RNA metabolism in the cell nucleus .