EVIDENCE FOR COORDINATE REGULATION OF THE A-SYSTEM FOR AMINO-ACID-TRANSPORT AND THE MESSENGER-RNA FOR THE ALPHA-1-SUBUNIT OF THE NA+,K+-ATPASE GENE IN CHINESE-HAMSTER OVARY CELLS

Previous work suggested that the structural gene for the A system transporter and the mRNA for the alpha-subunit of the Na+, K+ -ATPase in Chinese hamster ovary cells CHO-K1 [wild type (WT)] are coordinately controlled by regulatory gene R1. This conclusion was based on analysis of a mutant for the A system, ala(r)4. This mutant had a constitutive level of A system transport activity equal to the level found in derepressed WT cells and a 4 times increase in abundance of the alpha-1 subunit of Na+, K+ -ATPase mRNA over that found in repressed WT. The level of Na+ per cell in ala(r)4 was not significantly greater than that found in the WT. To further characterize the likely coregulation of both genes, we have studied the A system activity and Na+, K+ -ATPase mRNA alpha-1-subunit levels in cells grown under various conditions that result in repression or derepression of the A system in the WT. System A activity increased up to 2-3 times the basal transport rate (repressed state) and Na+, K+ -ATPase mRNA alpha-1-subunit levels showed a 3-fold increase after amino acid starvation (derepressed state). These changes occurred along with a decrease in intracellular Na+ levels. N-Methyl-alpha-aminoisobutyric acid and beta-alanine, previously shown to be corepressors for the A system, prevented to a similar extent A system derepression and Na+, K+ -ATPase mRNA alpha-1-subunit accumulation. On the other hand, phenylalanine and lysine, amino acids that are not corepressors of the A system, failed to significantly prevent derepression of both genes. Hybrids between the WT and ala(r)4 have the phenotype of the WT when grown under repressed conditions. These results give further support to the proposition that both the A system transporter and mRNA for the alpha-1 subunit of the Na+, K+ -ATPase are coordinately controlled by regulatory gene R1 and elevated Na+ concentrations are not involved. No Na+, K+ -ATPase activity was detected in derepressed cells. Activity was restored by the addition of monensin. However, this activity was no greater than that obtained in repressed cells. Indications are that the reduced Na+ content in derepressed cells inhibits Na+, K+ -ATPase activity and that conditions that favored derepression do not allow for de novo synthesis of the Na+, K+ -ATPase.