摘要：

As a prerequisite for a study of the transport and functional forms of messenger RNA and ribosomes in cultured L cells, a method has been developed for preparing cytoplasmic particles rid of the nuclear contaminants routinely found with conventional procedures. This method, which uses a low concentration of nonionic detergent and avoids mechanical disruption, was used to obtain polyribosomes that were demonstrably free of co-sedimenting ribonucleoprotein. Polyribosomes from cells pulse-labeled with [ 3H]uridine were analyzed by sucrose gradient sedimentation and equilibrium banding in CsCl before and after dissociation with EDTA. Newly synthesized messenger RNA's were released from the polyribosomes as a polydisperse array of ribonucleoprotein complexes (mRNP) with a relatively uniform RNA to protein ratio. The protein, termed m-protein, comprises about 60% of the weight of the complex. The mRNP released from polyribosomes was demonstrated to be similar in several respects to other RNP complexes in the cytoplasmic extracts which exist free of attached ribosomes. A more detailed examination of these latter structures indicated that roughly three-quarters of their protein could be reversibly removed by exposure to 0·55 m-LiCl-0·01 m-Mg 2+, a condition which removes very little protein from the ribosomal subunits. Previously we have demonstrated that new ribosomal subunits entering the cytoplasm contain more protein than the subunits in the steady-state pool. The present experiments indicate that this extra protein, termed accessory protein, is released from the subunits when they are incorporated into polyribosomes.

展开