To modify the lac permease of Escherichia coli for fluorescence spectroscopy, six tryptophan residues at positions 10, 33, 78, 151, 171, and 223 were first replaced individually with phenylalanine by using oligonucleotide-directed site-specific mutagenesis. None of the tryptophan residues is critical for activity, as evidenced by the finding that the mutant permease molecules catalyze lactose/H+ symport almost as well as wild-type permease. Subsequently, a permease molecule was designed in which all of the tryptophan residues were replaced with phenylalanine. Remarkably, the lac permease harboring all six mutations catalyzes active lactose transport about 75% as well as wild-type permease. The fluorescence emission spectrum of purified wild-type permease solubilized in octyl beta-D-glucopyranoside and phospholipid exhibits a broad maximum centered at 350 nm, and the peak is almost completely absent from the spectrum of permease devoid of tryptophan. Furthermore, a new maximum centered at about 306 nm is apparent in the spectrum of the modified permease, suggesting that tyrosine fluorescence in the native protein is quenched by internal energy transfer to tryptophan residues. By using site-directed mutagenesis to replace specified residues in the molecule without tryptophan, it should now be possible to utilize tryptophan fluorescence spectroscopy to study static and dynamic aspects of permease structure and function.