Interaction between the Different Domains of Aminoacyl‐tRNA and the Elongation‐Factor‐Tu · Kirromycin Complex

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Abstract
In this work, we have studied the effect of aa‐tRNA and derived 3′ aminoacylated fragments on the EF‐Tu GTPase in the presence of kirromycin, using two systems: without and with ribosomes. The aa‐tRNA fragments were obtained by enzymatic digestion. Procedures for the enzymatic preparation of C‐A‐Val and Val‐tRNA1Val 3′ half molecule, as well as a purification method for short 3′ aminoacylated fragments based on the amino group charge, were newly developed for this work. Aminoacyl‐adenosine was found to be able to stimulate the EF‐Tu · kirromycin GTPase, but only to a very small extent. Increasing the length of the aminoacylated fragments increased the stimulatory effect as follow: A‐Val ≪ C‐A‐Val < 3′ valyladenosine dodecanucleotide ≪ Val‐tRNA1Val 3′ half molecule < Val‐tRNA1Val. The presence of ribosomes did not affect the order of effectiveness, but increased the basic GTPase activity of EF‐Tu · kirromycin and the stimulation by aa‐tRNA, its 3′ half molecule and even more by its 3′ short fragments. The effect of aa‐tRNA and derived 3′ fragments in the absence of ribosomes was not influenced by MgCl2 concentrations of 5–30 mM whereas, in the presence of ribosomes, low concentrations of MgCl2 (5 mM) greatly reduced the stimulation of aa‐tRNA and, to a lesser extent, also the effect of the C‐C‐A‐aa as well as the basic activity of the EF‐Tu · kirromycin GTPase.The extent of the stimulation by aa‐tRNA, and even more by C‐C‐A‐aa, depends on the nature of the amino acid. Among the aminoacyl side chains tested (Arg‐, Phe‐, Val‐, Met‐, Leu‐, Lys‐) arginine was found to be the most active and leucine the least.Our results show that (a) the 3′ aminoacylated extremity is of prime importance for the stimulation of the EF‐Tu GTPase, (b) in the 3′ extremity there are critical sequences for the interaction with EF‐Tu and (c) other domains of the aa‐tRNA molecule are capable of influencing this reaction: one of the most important is the region including the TΨC loop and stem.

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