INHIBITION OF CHLOROPLAST PROTEIN-SYNTHESIS BY KIRROMYCIN - ACTION OFTHE ANTIBIOTIC ON ELONGATION-FACTOR TU

The antibiotic kirromycin inhibits protein biosynthesis in chloroplast s isolated from spinach (Spinacia oleracea) leaves and polyphenylalani ne synthesis in cell-free systems containing elongation factors from c hloroplasts. The mechanism of the inhibition is the same as that previ ously observed in Escherichia coli: the antibiotic blocks the release of elongation factor Tu (EF-Tu) from ribosome after GTP hydrolysis. Th e action of kirromycin on the interaction EF-Tu from spinach chloropla sts (EF-Tu(chl)) with GTP and GDP is also similar to that observed wit h EF-Tu from E. coli (EF-Tu(coli)): it increases greatly the affinity of EF-Tu for GTP and the rate of GDP-GDP and GTP-GDP exchange reaction s. In the presence of kirromycin the EF-Tu(chl). GDP complex becomes a ble to bind aminoacyl-tRNA even in the absence of ribosomes. This effe ct is likely responsible for the block of EF-Tu . GDP release from the ribosome during protein biosynthesis. The ability of the antibiotic t o activate the GTPase catalytic centre of EF-Tu(coli) was not observed in the case of EF-Tu(chl). The partial stimulation of the endogenous GTPase activity of EF-Tu(chl) appears to be due only to the antibiotic effect on the interaction of the elongation factor with GTP.