[Ribonucleoproteins containing mRNA in the cytoplasm of mouse plasmacytoma cells]

The rapidly labelled postribosomal ribonucleoprotein (RNP) found in the cytoplasm of mouse plasmacytoma cells were investigated. It has been shown that 45S and 80S particles contain relatively high molecular weight (approximately 12-17S) pulse-labelled RNA similar to the polyribosomal mRNA. No other postribosomal RNP was found which would contain an RNA with similar sedimentation characteristics. In CsC1 density gradients, the postribosomal RNP gives two peaks. One of them, the rapidly labelled component (rho 1.52 g/cm3) is found only in 45S RNP. The other rapidly labelled component (rho 1.36-1.41 g/cm3) is revealed in all investigated regions of sucrose gradients. The latter contains relatively low molecular weight RNA (approximately7-9S). These RNP are supposed to be informosome-like particles. The components with a buoyant density of 1.52 g/cm3 may represent an mRNP-45S subparticles complex. The rapidly labelled mRNA of 80S particles is released after EDTA treatment in the form of mRNP with a buoyant density of 1.45-1.47 g/cm3.