Regulation of pheA expression by the pheR product in Escherichia coli is mediated through attenuation of transcription.

In Escherichia coli, the expression of the phenylalanine biosynthetic enzyme chorismate mutase/prephenate dehydratase, encoded by pheA, is elevated in strains carrying pheR mutants. By constructing a series of pheA''cat'lacZ fusions with different endpoints for deletions of the pheA regulatory DNA, the site of action of the pheR product on pheA expression was determined to be the pheA attenuator. Southern blot analysis of chromosomal DNA from a pheR374 strain showed it to carry a deletion of pheR and the flanking DNA on each side. This deletion resulted in a decrease of approximately 30% in the intracellular concentration of tRNA(Phe), the pheR product. The expression of the pheST operon, which encodes the two subunits of phenylalanyl-tRNA synthetase and which is also regulated by attenuation control involving phenylalanyl-tRNA(Phe), was increased 5-fold by the pheR374 allele. No effect of pheR on pheST expression was seen in a pheST(att-) strain. It was concluded that the elevated expression of pheA and pheST in pheR mutants is a consequence of a lower frequency of transcription termination in the attenuator caused by lower levels of phenylalanyl-tRNA(Phe).