Nucleolar and nuclear RNA synthesis during the cell life cycle in monkey and pig kidney cells in vitro

Control of specific gene expression examined in synchronized mammalian cells

The control of synthesis of the steroid-inducible enzyme, tyrosine aminotransferase (TAT), has been studied in synchronized cultures of HTC cells, an established line of rat hepatoma cells. During the mitotic and early G1 periods of the generation cycle of cells previously exposed to inducer, TAT synthesis is maximal in the absence of inducer, i.e., synthesis is constitutive. (By contrast, in random cells maximal TAT synthesis is always dependent upon the presence of the inducer.) After the third hour of G1 "G1[3]"), TAT synthesis is inhibited by a specific posttranscriptional repressor, the formation of which requires RNA synthesis. We speculate that this repressor is antagonized by the steroid inducer. The degradation of TAT during mitosis and G1 is constant and not affected by cycloheximide or actinomycin D in our experiments.

Synthesis and induction of tyrosine aminotransferase in synchronized hepatoma cells in culture

The generation cycle of an established line of rat hepatoma cells (HTC cells) was studied using synchronized cell techniques. Dexamethasone phosphate (Dex), a synthetic adrenal corticosteriod which induces tyrosine aminotransferase (TAT) in HTC cells randomly distributed in the cell generation cycle, did not affect the durations of the various phases of the cycle. The activities of TAT and several dehydrogenases, and the rates of general protein and RNA synthesis, were studied throughout the cycle.Dex, at 10(-5)M, when added to a synchronized cell population in the latter two thirds of G1 phase or anywhere in the S phase, induces TAT. During the period made up of G2, M, and early G1, Dex does not induce TAT. However, radioisotope incorporation into specifically immunoprecipitated TAT continues during mitosis, when general protein and RNA synthesis are decreased.

RNA synthesis in Chinese hamster cells. I. Differential synthetic rate for ribosomal RNA in early and late interphase

The incorporation of methionine-methyl-(14)C into 18S ribosomal RNA of cultured Chinese hamster ovary cells in early and late interphase has been determined by zone-sedimentation analysis of phenol-extracted RNA preparations. Synchronized cell cultures were prepared for these studies by thymidine treatment and by mechanical selection of mitotic cells. The specific activity of 18S RNA labeled in late interphase was found to be 1.1-1.2 times that of 18S RNA labeled in early interphase. Upon correction for increase in RNA mass, the rate of methylation of 18S RNA in late interphase is about 1.9 times that in early interphase.