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J Biol Chem, Vol. 273, Issue 7, 3871-3877, February 13, 1998

Regulation of the Activity of Chloroplast Translational Initiation Factor 3 by NH₂- and COOH-Terminal Extensions

Nan-Jun Yu and Linda L. Spremulli^§

From the  Department of Chemistry and ^§ Lineberger Comprehensive Cancer Research Center, University of North Carolina, Chapel Hill, North Carolina 27599-3290

The mature form of the chloroplast translational initiation factor 3 (IF3_chl) from Euglena gracilis consists of an internal region homologous to prokaryotic IF3 flanked by long NH₂- and COOH-terminal extensions. Sequences in these extensions reduce the activity of the homology domain in promoting initiation complex formation with chloroplast mRNAs and 30 S ribosomal subunits. A series of deletions of the NH₂- and COOH-terminal extensions of IF3_chl were constructed and tested for their effects on the activity of the homology domain. About half of the inhibitory effect arises from sequences within 9 residues of the junction between the NH₂-terminal extension and the homology domain. The remaining inhibitory effect is the result of sequences in the COOH-terminal extension. The equilibrium constant governing the binding of the homology domain of IF3_chl to 30 S subunits is estimated to be 1.3 × 10⁷ M¹. Sequences close to the junction of the NH₂-terminal extension and the homology domain reduce this binding constant about 10-fold. Sequences in the COOH-terminal extension have a similar negative effect. The negative effects of these two regions are cumulative, resulting in a 100-fold reduction of the binding constant. The 9 residues at the NH₂-terminal extension effectively prevent the proofreading activity of IF3_chl. The entire COOH-terminal extension reduces the proofreading ability by about half. These results are discussed in terms of the proposed three-dimensional structure of the homology domain of IF3_chl.

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