Replication of viral RNA, 8. Studies on the enzymatic mechanism of replication of MS2 RNA.

postmicrosomal fraction contained rapidly labeled mRNA, which was also found to exist in a sedimentable form; newly made mRNA might thus be transported to its site of function in association with subcellular particles. Summary.-RNA from spleen and lymph nodes of the rat has been characterized by a study of its sedimentation properties, kinetics of radioactive labeling, nucleotide composition, and template activity. Immunization results in an unusually rapid rate of synthesis of rRNA and rRNA precursors, which account for a large part of newly synthesized RNA of these tissues. The first newly made RNA to reach the microsomes is a fraction of low molecular weight with the characteristics of mRNA. There is also evidence for the existence in the cell of mRNA of higher mol wt, which probably corresponds to mRNA as it is synthesized in the nucleus.' Newly made rRNA is transferred in the cytoplasm as subcellular particles; it appears very rapidly on the microsomes, 18S RNA preceding 30S RNA.