Ribonucleoprotein Particles Containing I) the Cellular Slime Mold Dictyostelium di

As in higher eukaryotes, heterogeneous nuclear RNA (hnRNA) in the cellular slime mold Dictyo- stelium discoideum is associated with proteins in the form of ribonucleoprotein particles. Mixing experiments with deproteinized hnRNA establish that the nuclear ribo- nucleoprotein particles are not formed artificially during isolation. In contrast to comparable imaterial from mam- malian cells (polydisperse, 40-250 S), Dictyostelium heterogeneous nuclear ribonucleoprotein particles sedi- ment at only 55 S in sucrose gradients, possibly reflecting the smaller size of slime mold hnRNA relative to the large hnRNA found in higher eukaryotes. The RNA of the nu- clear 55S ribonucleoprotein particles is shown to be hnRNA by virtue of its size (15S), content of polyadenylate se- quences, and hybridization kinetics at DNA excess. The hnRNA-associated proteins are electrophoretically com- plex and have molecular weights between 20,000 and 150,000. In 0.35 M NaCl most of the proteins are released from the hnRNA. However, a single protein of 72,000- 74,000 molecular weight remains bound, as indicated by