pheAo mutants of Escherichia coli have a defective pheA attenuator

Two classes of mutants affecting the regulation of pheA expression in Escherichia coli have been reported previously: trans-acting mutants involving the locus pheR, and cis-acting mutants involving the locus pheAo. The effects of these mutants have been found to be mediated through one regulatory mechanism. The gene pheR has been shown to encode tRNA(Phe) (Gavini, N., and Davidson, B. E. (1990) J. Biol. Chem. 265, 21527-21531). In this paper we report the cloning and nucleotide sequencing of the promoter-attenuator regions from two of the cis-acting mutants pheAo351 and pheAo352. Both pheAo351 and pheAo352 contained a G:C to A:T base pair transition, at different positions in the 3:4 stem of the pheA attenuator terminator. Since these changes would destabilize the G:C stem of the attenuator terminator we propose that the enhanced expression of pheA observed in the pheAo mutants is due to increased transcription readthrough at the defective attenuator terminator.