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J Biol Chem, Vol. 273, Issue 7, 3871-3877, February 13, 1998
and
§
From the The mature form of the chloroplast translational
initiation factor 3 (IF3chl) 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 IF3chl
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 IF3chl to 30 S subunits is
estimated to be 1.3 × 107
M
Department of Chemistry and
§ Lineberger Comprehensive Cancer Research Center,
University of North Carolina, Chapel Hill, North Carolina
27599-3290
1. 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
IF3chl. 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 IF3chl.
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