Fidelity of ribosomal ribonucleic acid synthesis by nucleoli and nucleolar chromatin.

Isolated nucleoli, nucleolar chromatin, and nucleolar DNA were used as templates for DNA synthesis in appropriately supplemented systems in which RNA polymerases other than RNA polymerase I were blocked by alpha-amanitin. With the aid of nucleotide analysis, DNA-RNA hybridization, and homochromatography fingerprinting, it was found that isolated nucleoli and nucleolar chromatin serve primarily as templates for synthesis of rRNA. However, the products formed with purified nucleolar DNA as a template do not contain the specific rRNA oligonucleotides nor are they appreciably hybridized to the rDNA region on cesium chloride gradients. These results indicate that whole nucleoli and nucleolar chromatin contain control mechanisms that restrict readouts by RNA polymerase I of nucleolar DNA to rDNA.     

Reversible inhibition of ribosomal RNA synthesis in HeLa by lucanthone (Miracil D) with continued synthesis of DNA-like RNA

Ribosomal RNA synthesis was selectively inhibited in HeLa cells by lucanthone, a clinically useful schistosomicide which shares many of the properties of Actinomycin D. Synthesis of DNA-like RNA continued during complete inhibition of ribosomal RNA synthesis. Under these conditions newly synthesized DNA-like RNA accumulated normally in polyribosomes of the cell cytoplasm; most of it appeared to be messenger RNA. DNA synthesis was partially inhibited by lucanthone but protein synthesis was undisturbed. Synthesis of ribosomal RNA promptly resumed after removal of lucanthone and cell survival was not affected if exposures to the drug were limited to two hours.     

Oligo(A)-stimulated Tetrahymena rDNA synthesis in vito

The synthesis of Tetrahymena rDNA has been examined using purified DNA polymerase and partially purified preparations of homologous replication enzymes (fraction IV). DNA synthesis with purified DNA polymerase alone was less than that with fraction IV enzymes. This suggested that there were additional factors in fraction IV other than DNA polymerase which contributed to or enhanced rDNA synthesis in vitro. Neither hybridization of rDNA with Tetrahymena ribosomal RNA nor preincubation of rDNA with homologous or heterologous RNA polymerase served to stimulate in vitro synthesis by fraction IV enzymes. However, when rDNA was hybridized with oligoriboadenylate, DNA synthesis using fraction IV was stimulated approximately 4- to 4.5-fold over 150 min of incubation, relative to a similarly treated but unhybridized rDNA control. Using oligoriboadenylate-hybridized EcoR1 and HindIII restriction fragments of rDNA to localize the synthesis most of the in vitro synthesis occurred with a 2.4 X 10(6) Mr fragment encompassing the centre of the rDNA molecule. The approach of hybridizing a synthetic homooligoribonucleotide primer to double-stranded DNA should prove to be of general applicability in designing similar template-primers in other systems for the purpose of isolating replication proteins.     

Vesicular stomatitis virus infection reduces the number of active DNA-dependent RNA polymerases in myeloma cells.

Infection of mouse myeloma (MPC-11) cells with vesicular stomatitis virus resulted in rapid loss in activity of cellular RNA polymerases associated with nuclear chromatin. No RNA polymerase inhibitor could be detected in extracts of infected cell nuclei. Reconstitution experiments with solubilized RNA polymerases dissociated from chromatin of infected and uninfected cells demonstrated that vesicular stomatitis viral infection did not affect the ability of the polymerases to function on endogenous or exogenous templates; nor did infection alter the template capability of the chromatin. Measurement of the number of actively growing RNA chains revealed that infected cell nuclei contained fewer active polymerase units; however, the rates of RNA chain elongation were the same in nuclei from infected and uninfected cells. Quantitation of the number of polymerase units active in nuclear chromatin revealed that the alpha-amantin-sensitive polymerase II was more severely reduced by viral infection than were polymerases I and III.