The Lrp regulon of Escherichia coli.

Abstract

Bacteria respond to changes in their environment by altering gene expression. Lrp, the leucine-responsive regulatory protein, mediates changes in gene expression in response to the presence or absence of leucine in the environment. Our goals have been to define the set of genes regulated by Lrp (the Lrp regulon), and then to fill in some of the details of regulation for one regulated system, the gltBDF operon encoding glutamate synthase. We have compared the patterns of protein expression in strains containing or lacking a functional Lrp protein. At least 33 polypeptides show altered expression when the two strains are compared; about half are positively regulated by Lrp and the other half are negatively regulated. We have identified several leucine-unresponsive proteins in the Lrp regulon, including the porins OmpC and OmpF, glutamine synthetase and glutamate synthase. Experiments with lacZ fusions to the gltBDF promoter have shown that the transcription of this operon is 44-fold lower in lrp strains than in $lrp\sp+$ strains. In $lrp\sp+$ strains, the presence of exogenous leucine leads to a 2-fold reduction in the transcription of the gltBDF operon, encoding glutamate synthase, indicating that the regulation of gltBDF by Lrp is not independent of leucine. Gel mobility shift assays have identified an Lrp binding region upstream of the gltBDF operon. We have compared the binding of Lrp to the DNA of the gltDBF and ilvIH promoters. The ilvIH operon is positively regulated by Lrp in a leucine-sensitive manner, with the presence of exogenous leucine leading to a 5.5-fold reduction in expression. The results of this comparison suggest that Lrp is a positive regulator of gltBDF and ilvIH in the presence and absence of leucine, and that the concentration of Lrp in the cell determines which genes and operons are sensitive to leucine. In lrp strains, glutamate synthase activity is nearly absent, and these strains are unable to respond normally to nitrogen starvation. One of the functions of Lrp may be to distinguish between conditions where nitrogen is present as ammonia and conditions where nitrogen is present in more complex forms.