Wirkung von Metalaxyl auf die Synthese von RNA, DNA und Protein inPhytophthora nicotianae

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Abstract
Metalaxyl is used to control diseases caused by fungi of the order of the Perenosporales. The action of this fungicide on nucleic acid and protein synthesis in liquid cultures of P. nicotianae was investigated. The uptake of 32P, 3H-uridine, 3H-thymidine and 14C-leucine as precursors of nucleic acid and protein synthesis by the mycelium was not inhibited by metalaxyl. RNA synthesis as indicated by 3H-uridine incorporation was strongly inhibited (.apprx. 80%) by 0.5 .mu.g/ml of metalaxyl. The inhibition was visible already a few minutes after addition of the toxicant. Since the inhibition of incorporation of 3H-thymidine into DNA and of 14C-leucine into protein became significant 2-3 h later, it was concluded that metalaxyl primarily interfers with RNA synthesis. Synthesis of rRNA is more affected (> 90%) than that of tRNA (.apprx. 55%) and poly(A)-containing RNA. Since in the presence of actinomycin, in contrast to metalaxyl, protein synthesis is inhibited immediately as a consequence of complete inhibition of RNA synthesis and of the short life-time of mRNA, it is also evident that mRNA synthesis is less strongly inhibited, at least during the early period of metalaxyl action. The molecular mechanism of metalaxyl inhibition of the transcription process remains open. The fungicide did not inhibit the activity of a partially purified RNA polymerase isolated from the fungus, but the RNA synthesis (14C-UTP-incorporation) by a cell homogenate and by isolated nuclear fractions was inhibited significantly. Possibilities of the molecular action of metalaxyl are discussed. The RNA synthesis of some plant systems (cell cultures of Lycopersicon peruvianum, isolated nuclei from the same cell cultures, purified RNA polymerase from Spinacia oleracea chloroplasts) was not inhibited by metalaxyl, not even at high concentrations.