Effector overlap between the lac and mel operons of Escherichia coli: Induction of the mel operon with β-galactosides

Abstract

The lac (lactose) operon (which processes β-galactosides) and the mel (melibiose) operon (which processes α-galactosides) of Escherichia coli have a close historical connection. A number of shared substrates and effectors of the permeases and regulatory proteins have been reported over the years. Until now, β-thiogalactosides like TMG (methyl-β-D-thiogalactopyranoside) and IPTG (isopropyl-β-Dthiogalactopyranoside) have not generally been considered to be inducers of the mel operon. The same is true for β-galactosides such as lactose [β-D-galactopyranosyl-( 1→4)-D-glucose], which is a substrate but is not itself an inducer of the lac operon. This report shows that all three sugars can induce the mel operon significantly when they are accumulated in the cell by Lac permease. Strong induction by β-thiogalactosides is observed in the presence of Lac permease, and strong induction by lactose (more than 200-fold) is observed in the absence of β-galactosidase. This finding calls for reevaluation of TMG uptake experiments as assays for Lac permease that were performed with mel+ strains.