Alanine at position 294 (Ala294) within the motif 3 consensus of Escherichia coli phenylalanyl-tRNA synthetase alpha subunit has previously been implicated as a determinant of amino acid specificity. To characterize the role of Ala294, the catalytic effects of amino acid replacements at this position were tested with purified wild-type and mutant phenylalanyl-tRNA synthetases. We show that Ala294 is involved in amino acid binding and that it influences specificity as a determinant of binding pocket size. Replacement of Ala294 by either glycine or serine, thereby increasing or decreasing the size of the binding pocket, respectively, reduces affinity for phenylalanine. The Gly294 mutant shows a relaxed specificity toward synthetic para-halogenated phenylalanine analogues, the apparent dissociation constant Km increasing in direct relation to an increase of the van der Waals radius of the para group, thus confirming the role of position 294 in determining amino acid binding pocket size. For the substrate analogue p-chlorophenylalanine, attachment to tRNA and in vivo incorporation into cellular protein by the Gly294 mutant were demonstrated. Tyrosine activation was also improved with this mutant, but the resulting enzyme-Tyr-adenylate complex was rapidly hydrolyzed, indicating the presence of a proofreading mechanism in E. coli phenylalanyl-tRNA synthetase.