MODIFICATION OF RIBONUCLEIC ACID SYNTHESIS IN ISOLATED RAT LIVER NUCLEI BY LOW SALT CONCENTRATIONS AND SPECIFIC DIVALENT CATIONS

Studies of RNA synthesis by isolated and intact rat liver nuclei suggest that at least two types of RNA products can be obtained: one with high GC content and preferential localization in the nucleoli; the other with high AU content and mainly extranucleolar localization. These different nuclear activities are assayed by adding NaCl or (NH4)2SO4 at very low concentrations (0.06 ionic strength) and using either Mg2+ or Mn2+ as divalent cations. Since both salts have similar effects at low concentrations, the selective factor appears to be the divalent cation. NaCl and Mg2+ in the incubating medium stimulate the synthesis of RNA species enriched in GC content. The RNA synthesized under these conditions resembles ribosomal RNA. NaCl and Mn2+ in the incubating medium stimulate the synthesis of RNA species enriched in AU content. (NH4)2SO4 has similar effects, but NaCl is a better salt than (NH4)2SO4 for obtaining a clear distinction of both activities. The optimum pH is similar for both activities. These results indicate that both the nature and the site of RNA synthesis can be controlled in isolated and intact rat liver nuclei. The selective activation of extranucleolar synthesizing sites and a newly synthesized RNA enriched in adenylic and uridilic acid contents can be obtained without high salt concentrations in the incubating medium. It is inferred from these results that it is not necessary to add high (NH4)2SO4 concentrations to the incubating medium to induce the synthesis of a DNA-like RNA. © 1969.