Evidence that tryptophanyl transfer ribonucleic acid is not the corepressor of the tryptophan operon of Escherichia coli

Abstract

When a growing culture of a tryptophan auxotroph of Escherichia coli is transferred to a tryptophan-free medium, the bacteria exhaust their supply of Trp-transfer ribonucleic acid (tRNA). Under these conditions transcription of the trp operon is derepressed. When l-tryptophan, dl-4-methyltrytophan, dl-6-methyltryptophan, or dl-7-azatryptophan is added to a tryptophan-starved culture, charging to tRNA(Trp) can be detected with each of these compounds except 6-methyltryptophan. Under these conditions, transcription initiations on the trp operon are repressed completely by tryptophan, 4-methyltryptophan, and 6-methyltryptophan, but only slightly by 7-azatryptophan. If a culture of a bacterium containing an altered tryptophanyl-tRNA synthetase (trpS(-)) is starved for tryptophan in the same manner, tRNA(Trp) is uncharged, but transcription of the operon is derepressed to only about one-third the level observed in trpS(+) cultures. When tryptophan, 4-methyltryptophan, 6-methyltryptophan, or 7-azatryptophan is added to a tryptophan-starved trpS(-) culture, tRNA(Trp) is charged with tryptophan but apparently not with the analogues. However, tryptophan, 4-methyltryptophan, and 6-methyltryptophan completely repress transcription initiations, whereas 7-azatryptophan derepresses transcription initiations to approximately the level of partial repression observed in trpS(+) cultures in the presence of 7-azatryptophan. When tryptophan is added to a tryptophan-starved trpS10110 culture, there is a 2-min delay before tRNA(Trp) appears to be charged. However, under these conditions, transcription initiations on the trp operon are repressed immediately by the addition of tryptophan. We interpret these results as indicating that Trp-tRNA is not the corepressor of the trp operon.