Mutations affecting tRNA Trp and its charging and their effect on regulation of transcription termination at the attenuator of the tryptophan operon

Abstract

A mutant altered in the sole structural gene for tRNA Trp produces sevenfold elevated levels of trp operon enzymes and messenger RNA. These increases are probably due to a reduced frequency of transcription termination at the attenuator region of the operon. A normal termination frequency is observed in this mutant, however, when conditions are imposed that should permit an increase in the extent of charging of the mutant tRNA Trp. Similarly, a mutant with an altered tryptophanyl-tRNA synthetase exhibits increased expression of the operon; this increase is also eliminated by conditions which should allow charging of a greater fraction of cellular tRNA Trp. These observations suggest that the extent of charging of tRNA Trp rather than the availability of tryptophan is the determinative factor in regulation at the attenuator. A second mutation which alters tRNA Trp, but which is not in the tRNA Trp structural gene, also increases expression of the operon. Conditions which should promote charging of this tRNA Trp do not overcome the regulatory defect. Analyses were performed with various trpT heterozygotes in an attempt to determine whether charged or uncharged tRNA Trp is the active species in the regulation of transcription termination at the attenuator. The findings obtained suggest that both species influence regulation at the attenuator.