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Sedimentation analyses in sucrose-density gradients have been performed on RNA components in ribonucleoprotein particles occurring during the course of ribosome formation in the “shift-up” culture of Escherichia coli . It was found that the 18-S chloramphenicol particle, 28-S 5-fluorouracil particle and 30-S ribosomal subunit contained solely 16-S ribosomal RNA component, while 25-S chloramphenicol particle, 32-S 5-fluorouracil particle, 38-S particle, 43-S particle and 50-S ribosomal subunit contained only 23-S ribosomal RNA. From these observations, together with those reported previously 1 , a tentative scheme for the formation of ribosomes in E. coli “shift-up” culture has been presented.

In D. discoideum, RNA and protein syntheses were not restricted to any particular stage of morphogenesis but continued throughout. However, the contents of RNA and protein decreased progressively during development. Sucrose density-gradient analysis showed that polysomes varied in size throughout development and that the amounts of polysomes decreased during development. The sedimentation patterns of rapidly labeled RNA in polysomes differed significantly at three different stages of development. Actinomycin D prevented development of the amoebae and incorporation of 3H-uridine into RNA of whole cells when added at an early stage of morphogenesis. It did not affect incorporation of 14C-amino acids into protein. Cycloheximide inhibited overall morphogenesis and 14C-amino acid incorporation at almost all stages of development. The relationship between RNA and protein syntheses during morphogenesis is discussed.

Experiments were performed to determine the mode of action of 5-fluorouracil on the formation of Escherichia coli ribosomes. The following results were obtained. (1) The CM (chloramphenicol)-particles containing 5-fluorouracil in their RNA were shifted in the presence of 5-fluorouracil to the FU (5-fluorouracil)-particles, while the CM-particles not having had 5-fluorouracil were converted to mature ribosomes even in the presence of 5-fluorouracil. (2) The proteins formed in the course of accumulation of the FU-particles were not utilized for the subsequent conversion of the FU-particles to the 50-S and 30-S ribosomal subunits when 5-fluorouracil was withdrawn from the medium. Protein available for this purpose was formed only after the removal of 5-fluorouracil. (3) 5-Fluorouracil incorporated into RNA seems to be replaced by uracil when cells were further incubated in the uracil-rich medium. At the same time, the FU-particles were transformed into mature ribosomes. It is concluded that the 5-fluorouracil incorporated into RNA, but not free 5-fluorouracil, causes the accumulation of FU-particles, which inhibits the synthesis of certain protein components of the ribosomes, thus preventing the formation of mature ribosomal particles.