Ribonucleoprotein Particles in HeLa Cells. The Contamination of the Postmitochondrial-Cytoplasmic Fraction from HeLa Cells with Ribonucleoprotein Particles of Mitochondrial Origin

We have investigated some properties of a class of cytoplasmic ribonucleoprotein particles in HeLa cells. After 3.5 h of labelling in the presence of 0.04 μg/ml actinomycin D to inhibit rRNA synthesis, about 20% of the newly synthesized cytoplasmic RNA, that sediments faster than 7 S, is contained in two types of particles sedimenting at 45 S and 30 S in sucrose gradients. We have examined this material in more detail with the following results. 1 The fixed particles band as two sharp peaks of density o= 1.440 g/cm3 for the 45-S and density o= 1.460 g/cm3 for the 30-S material. We call them the 45-S-M and 30-S-M particles, respectively. 2 The RNA of the 45-S-M and 30-S-M particles sediment in sucrose gradients as sharp peaks at 16 S and 12 S, respectively. 3 The synthesis of these two RNA species is inhibited by ethidium bromide, but resistant to low concentrations of actinomycin D. Two RNA species with these drug sensitivities and sedimentation characteristics have been found in the mitochondria of HeLa cells [1]. 4 If cell homogenization and fractionation are carried out without detergents, the 45-S-M and 30-S-M particles are quantitatively bound to mitochondria. 5 Under most of the conditions commonly used for cell lysis and fractionation, these particles are extracted into the postmitochondrial-cytoplasmic fraction.