Inhibition of Photoperiodic Induction by 5-Fluorouracil

In this paper it will be shown that photoperiodic induction of the cocklebur, a short day plant, is inhibited by the pyrimidine 5-fluorouracil ( 5-FU). The studies of other workers have shown that 5-FU inhibits the growth of various kinds of cells and tissues by suppressing the formation of thymidine, and that application of thymidine relieves the inhibitory effects of 5-FU. In these cases 5-FU is an inhibitor of DNA synthesis (2, 4, 5). In other cases, however, 5-FU inhibits RNA synthesis. Thus 5-FU inhibits the production of tobacco mosaic viral RNA by tobacco leaves ( 1). This inhibition, which is relieved neither by thymidine nor by uracil, is associated with incorporation of 5-FU into the viral RNA (3). In. hibition of photoperiodic induction by 5-FU is not reversed by either thymidine or uracil but is reversed by orotic acid (6) which is known to be an intermediate in the biogenesis both of uridine and cytidine and of deoxycytidine and thymidine. Reversal of 5-FU inhibition by orotic acid suggests the hypothesis that the inhibition is related in some way to suppression of nucleic acid metabolism.