Uridine incorporation and RNA synthesis during stimulation of lymphocytes by PHA

Abstract

The transformation of lymphocytes by PHA is accompanied by a marked increase in the rate of incorporation of 3H-uridine into RNA. Nucleosides seem to be the major source of pyrimidine nucleotides in this system, but evidence is presented that uridine incorporation into RNA is limited by the rate of phosphorylation of uridine by uridine kinase. There is no simple relationship between uridine incorporation into RNA and the rate of RNA synthesis. Uridine kinase activity, assayed in vitro, increases in PHA-stimulated lymphocytes, but this increase is not necessary for at least the initial increase in the rate of uridine incorporation. Studies in which lymphocyte growth was limited by low concentrations of actinomycin or prostaglandin E 1 indicated that the rise in uridine kinase activity was dependent on the increase in uridine incorporation rather than vice-versa. We conclude that the amount of uridine kinase normally present in the lymphocyte does not limit the rate of phosphorylation of uridine, and that uridine kinase activity in the intact cell is subject to regulation by some factor other than the enzyme concentration, possibly the intracellular concentration of pyrimidine triphosphates. The implications of these results for studies on the changes in RNA metabolism during the stimulation of lymphocytes by PHA are discussed. Such considerations may be widely applicable to studies of the incorporation of precursors into nucleic acids in eucaryotic cells.