Regulation of branched-chain amino acid transport in Escherichia coli by attenuation and repression.

Abstract

The regulation of the LIV-I transport system has been investigated. The role of the leucine-coding leader peptide gene of the co-cistronic leucine-isoleucine-valine binding protein gene, livJ, has been examined by site-directed mutagenesis and fusion of the regulatory segment of the livJ gene to the gene coding for $\beta$-galactosidase, lacZ. The results show that the leader peptide gene functions to regulate expression of livJ in the presence of exogenous leucine. A mutation that alters the leader peptide gene such that it no longer codes for leucine, or one that alters the translation initiation codon of this gene results in the loss of regulation of livJ by exogenous leucine by attenuation. Repression of the liv operons has been investigated. It has been shown that mutations in the lrp gene, which codes for the leucine-responsive regulatory protein, Lrp, affect regulation of the livJ and livK promoters. The livK gene codes for the leucine-specific binding protein, and is the first gene of an operon that codes for the membrane components common to both high affinity transport systems for leucine. This regulation is unusual, compared to previously examined promoters regulated by Lrp. In previous cases, the activity of Lrp has been shown to be antagonized by the addition of free leucine. In the liv operons, the activity of Lrp is stimulated by the addition of free leucine.