Mechanism of the spontaneous and initiation factor 3-induced dissociation of 30 S.aminoacyl-tRNA.polynucleotide ternary complexes

Gualerzi, C.; Risuleo, G.; Pon, C.

Journal of Biological Chemistry 254(1): 44-49


ISSN/ISBN: 0021-9258
PMID: 363719
Accession: 068527067

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The mechanism of the spontaneous and initiation factor 3 (IF-3)-induced dissociation of a 30 S .cntdot. N-acetylphenylalanyl-tRNA .cntdot. poly(U) ternary complex was analyzed in toto and in its partial reactions. Overall, the factor produces a substantial increase in the rates of formation and dissociation of the ternary complex and a shift of the equilibrium toward dissociation. The IF-3-dependent release of poly(U) from the 30 S ribosomal subunits accompanies the dissociation of N-acetylphenylalanyl-tRNA from ternary complexes but is not a prerequisite for the dissociation since the aminoacyl-tRNA can be released even when the polynucleotide is irreversibly cross-linked to the ribosome. Likewise, the binding of the polynucleotide to the 30 S subunits does not appear to be the step where IF-3 exercises its main stimulation since the binding of N-acetylphenylalanyl-tRNA to 30 S ribosomes having a covalently bound polynucleotide is increased several-fold in the presence of the factor. The formation and the dissociation of the ternary complex proceed via 2 intermediate binary complexes, 30 S .cntdot. poly(U) and 30 S .cntdot. N-acetylphenylalanyl-tRNA. The forward and back rate constants for the formation of these 2 binary complexes, and their association constants are increased by IF-3 which also acts as a kinetic effector on a reaction (probably a conformational transition) which is rate-limiting in the dissociation (and formation) of the ternary complex. The nature of this reaction is probably the same, in both spontaneous and IF-3-induced dissociation of the ternary complex, but IF-3 reduces the energy of activation of this process by approximately 5 kcal/mol.