Cytoplasmic RNA in immunoglobulin-secreting myeloma tumor cells: I. Identification and characteristics of a class of heterogeneous RNA

Abstract

A set of heterodisperse RNA molecules was identified in cytoplasmic extracts of a cultured strain of mouse myeloma cells (Strain MOPC 21). The molecules range in size from 10 5 to 10 6 daltons, as estimated from their electrophoretic mobilities on polyacrylamide gels. Most of the species are 4 · 10 5–8 · 10 5 daltons. Synthesis of two of these species is selectively inhibited by 1 μg/ml of ethidium bromide; these correspond in size to mitochondrial ribosomal RNA in HeLa cells. Synthesis of the other RNA molecules is not preferentially inhibited by ethidium bromide, suggesting a nuclear origin. Also, they are not affected by a concentration of actinomycin D which inhibits (nuclear) ribosomal RNA synthesis. The accumulation of radioactive precursor into this RNA is very rapid; after an initial lag incorporation proceeds on a mole basis at about 25 % of the rate of that into ribosomal RNA. There is probably significant turnover in this class of RNA. Molecules of similar size are found in polyribosomal fractions and in smaller cytoplasmic material. The heterogeneous RNA in these cells is distinct from that in a mouse lymphoma strain, but is not greatly different from that of a myeloma strain in which immunoglobulin is not detected. These results are discussed with reference to expected properties of a population of messenger RNA molecules.