Effect of 5-Fluorouracil on the synthesis of chloroplast and cytoplasmic ribosomal RNA in leaves of Nicotiana rustica

Abstract

Summary In leaves of Nicotiana rustica the formation of ribosomal RNA is inhibited by FU at concentrations higher than 90 μg FU per g fresh weight.The synthesis of low molecular weight RNA and D-RNA is inhibited at concentrations more than 350μg FU per g fresh weight.The synthesis of nuclear DNA is also inhibited at lower FU-concentrations presumably via inhibition of thymidylate-synthetase. Application of 300μg FU/g fresh weight prevents the formation of cytoplasmic rRNA by interfering with the maturation process of the RNA.While high molecular weight RNA (mol.wt.> 1.4 × 10 6 D) slightly accumulates or remains unaffected after pretreatment of tobacco leaves with the antimetabolite, the formation of 1.4 × 10 6 D ribosomal pre-RNA and of mature cytoplasmic rRNA (mol.wt.of about 1.3 and 0.7 × 10 6 D) is almost completely inhibited.At the same concentration the formation of chloroplast ribosomal RNA, i.e.the synthesis of 1.28 and 0.7 × 10 6 D ribosomal pre-RNA and their transformation into mature rRNA remains unaffected by the drug. This different sensitivity of chloroplast and cytoplasmic rRNA formation to FU cannot be explained by a different uptake of the analogue into nuclei and chloroplasts, because labelled 5-fluorouracil is incorporated into cytoplasmic as well as into chloroplast ribosomal RNA.