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Initiation factor IF3 contains two domains separated by a¯exible linker. While the isolated N-domain displayed neither af®nity for ribosomes nor a detectable function, the isolated C-domain, added in amounts compensating for its reduced af®nity for 30S subunits, performed all activities of intact IF3, namely: (i) dissociation of 70S ribosomes; (ii) shift of 30S-bound mRNA from`stand-by' to`P-decoding' site; (iii) dissociation of 30S±poly(U)±NacPhe-tRNA pseudoinitiation complexes; (iv) dissociation of fMet-tRNA from initiation complexes containing mRNA with the non-canonical initiation triplet AUU (AUUmRNA); (v) stimulation of mRNA translation regardless of its start codon and inhibition of AUUmRNA translation at high IF3C/ribosome ratios. These results indicate that while IF3 performs all its functions through a C-domain±30S interaction, the N-domain function is to provide additional binding energy so that its¯uctu-ating interaction with the 30S subunit can modulate the thermodynamic stability of the 30S±IF3 complex and IF3 recycling. The localization of IF3C far away from the decoding site and anticodon stem±loop of P-site-bound tRNA indicates that the IF3 ®delity function does not entail its direct contact with these structures.

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