| Abstract: | Guanosine 5'-O-(3-thio)triphosphate (GTP gamma S) is a good analog of GTP in the reactions leading to the formation of a peptide bond in protein biosynthesis. It forms binary and ternary complexes with elongation factor Tu (EF-Tu), and with EF-Tu and aminoacyl-tRNA (aa-tRNA). In addition, it stimulates aa-tRNA binding to ribosomes. Although GTP gamma S hydrolysis is more than three orders of magnitude slower than GTP hydrolysis, both reactions are dependent on the formation of a noncovalent complex (RS X TC) between mRNA-programmed ribosomes and ternary complex, and the complexes resulting from that hydrolysis are intermediates in peptide formation. The rate of dissociation of the ribosome X EF-Tu X GTP gamma S X aa-tRNA complex was determined from the rate of labeled peptide formation in the presence of an unlabeled ternary complex chase. This rate (2.2 X 10(-3) s-1) is similar to that determined previously (Thompson, R.C., and Karim, A.M. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 4922-4926) from the progress of GTP gamma S hydrolysis. The effects of temperature and polycation concentration on this rate constant and that for GTP gamma S hydrolysis are reported. The rate constants measured are consistent with a kinetic rather than thermodynamic limit on the accuracy of the aa-tRNA selection in vivo. |
