Eigenschaften einer Chloroplasten-RNA vom Molekulargewicht 0,5×106 aus Nicotiana rustica

[Effect of metalaxyl on the synthesis of RNA, DNA and protein in Phytophthora nicotianae]

Metalaxyl is used to control diseases caused by fungi of the order of the Perenosporales. We investigated the action of this fungicid eon nucleic acid and protein synthesis in liquid cultures of Phytophthora nicotianae. The uptake of 32P, 3H-uridine, 3H-thymidine and 14C-leucine as precursors of nuclei acid and protein synthesis by the mycelium was not inhibited by metalaxyl. RNA synthesis as indicated by 3H-uridine incorporation was strongly inhibited (about 80%) by 0.5 micrograms/ml of metalaxyl. The inhibition was visible already 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 hours later, we conclude that metalaxyl primarily interfers with RNA synthesis. Synthesis of ribosomal RNA is more affected (more than 90%) than that of tRNA (about 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. On the other hand, 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.

Processing of a chloroplast-translated membrane protein in vivo. Analysis of the rapidly synthesized 32 000-dalton shield protein and its precursor in Spirodela oligorrhiza

The 32 000-dalton (Da) shield protein regulating electron transport in Spirodela oligorrhiza is an integral chloroplast membrane polypeptide. It is rapidly synthesized, constituting a major chloroplast-translation product in vivo. Following in vitro translation of spirodela chloroplast RNA in a wheat germ system, a 33 500-Da polypeptide is produced. Synthesis of a 33 500-Da protein, associated with the chloroplast membrane, is also seen in vivo, within 2 min of pulse-labeling spirodela with radioactive amino acids. Comparative analyses among these polypeptides reveal: (a) all three are deficient in lysine residues; (b) the two 33 500-Da species have indistinguishable partial proteolytic digestion patterns while that for the 32 000-Da protein differs only slightly from them; (c) radioactivity from the 33 500-Da polypeptide is rapidly chased in vivo into the 32 000-Da protein, even in the presence of protein synthesis inhibitors. These results show the 33 500-Da proteins synthesized in vitro and in vivo to be the precursor form of the 32 000-Da shield protein in spirodela, with processing commencing only after completion of the precursor polypeptide chain and insertion into the membrane.