Pyrimidine Metabolism in Lemna minor

Cytidine deoxyriboside (Cdr), uridine deoxyriboside (Udr), and guanosine deoxyriboside (Gdr), induce quantitative bleaching of the fronds of Lemna minor (duckweed) during growth in continuous light on photoheterotrophic medium. Cdr-induced bleaching is not accompanied by a reduction in frond multiplication rate, but Udr- and Gdr-induced bleaching is. Bleaching by Cdr is fully prevented by thymidine (Tdr), cytidine (Cr), or uridine (Ur), but not by orotic acid (OA) which itself inhibits growth. Bleaching by Udr is not antagonized by Tdr, Cdr, Cr, Ur, or OA. The ability of Cdr to induce phenocopies of chlorophyll-deficient mutants in the absence of effect on growth rate is interpreted as indicating a functional compartmentation of pyrimidine metabolism between chloroplast and whole cell. On the assumption that Cdr induces bleaching by regulating the biosynthesis of deoxynucleoside triphosphates, and in analogy with the antagonism of fluorodeoxyuridine effects on growth by Tdr, Cr, or Ur, the suggestion is made that deoxycytidine is converted to thymidylate by a step other than that utilizing thymidylate synthetase.