RNA SYNTHESIS IN CULTURED CHICK EMBRYO CELLS IN GROWING AND CONFLUENT PHASES

RNA synthesis at the growing phase in monolayer cultures of chick embryo fibroblasts was compared with that at confluent phases by zonal sedimentation, base composition and hybridization experiments. The nuclei were isolated by treatment with Nonidet p‐40. The ratio of RNA/DNA in isolated nuclei was higher at the growing phase than that of confluent. The rate of RNA synthesis was reduced in the cells at confluent phase to 15.1% of that at the growing phase. The sucrose density gradient sedimentation pattern of nuclear RNA was on the whole the same in both phases. According to the distribution of 14 C‐uridine incorporated into nuclear RNA, 45S ribosomal precursor RNA was more distinct for the growing cell, while the radioactivities were found to be polydispersed, including the RNA which sedimented faster than 28S RNA in the cells at confluent phase. The base compositions and hybridization analyses indicated that ribosomal RNA was synthesized more actively in the growing cells. About 50% of newly synthesized RNA was ribosomal in the growing cells but 35% in the confluent. It was found that newly synthesized 18S and 28S ribosomal RNAs appeared in cytoplasm after 21 and 33 min lag periods respectively. These times were exactly same in both growing and confluent phases.