THE BN51 PROTEIN IS A POLYMERASE (POL)-SPECIFIC SUBUNIT OF RNA POL-III WHICH REVEALS A LINK BETWEEN POL-III TRANSCRIPTION AND PRE-RIBOSOMAL-RNA PROCESSING

The three eukaryotic nuclear RNA polymerases (Pol) contain common and unique subunits. Cloning of the unique Pol III subunit genes in yeast cells has revealed a potential homolog in the mammalian system, the BN 51 gene. The human BN51 gene was originally isolated as a suppressor o f a temperature-sensitive cell cycle mutant of BHK cells (tsBN51). Alt hough tsBN51 cells have a marked decrease in RNA Pol III activity at t he nonpermissive temperature, direct biochemical evidence for the BN51 protein being a human Pol III subunit was lacking. Using antibodies d irected against the BN51 protein, we show the following: (i) the BN51 protein copurifies with Pol III activity, (ii) Pol III activity can be specifically immunoprecipitated from HeLa nuclear extracts, and (iii) the immunopurified BN51 complex is active in restoring both nonspecif ic and promoter-specific Pol III activity. Our findings provide direct biochemical evidence for BN51 being a Pol III-specific subunit. Despi te the fact that BN51 is not a subunit of Pol I, the production of mat ure Pol I transcripts is inhibited in tsBN51 cells at the nonpermissiv e temperature. tsBN51 cells appear defective in processing the 32S pre cursor rRNA into mature 5.8S and 28S rRNA at the nonpermissive tempera ture. We surmise that ribosome assembly has halted because of the loss of Pol III transcripts. Thus, there is regulation of the synthesis of mature Pol I transcripts by a posttranscriptional mechanism based on the availability of Pol III transcripts.