Effector Overlap between the and Operons of Escherichia coli: Induction of the Operon with β-Galactosides.

The (lactose) operon (which processes β-galactosides) and the (melibiose) operon (which processes α-galactosides) of 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-d-thiogalactopyranoside) have not generally been considered to be inducers of the 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 operon. This report shows that all three sugars can induce the 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 strains. The typical textbook picture of bacterial operons is that of stand-alone units of genetic information that perform, in a regulated manner, well-defined cellular functions. Less attention is given to the extensive interactions that can be found between operons. Well-described examples of such interactions are the effector molecules shared by the and operons. Here, we show that this set has to be extended to include β-galactosides, which have been, until now, considered not to effect the expression of the operon. That they can be inducers of the operon as well as the operon has not been noted in decades of research because of the genetic background used in previous studies.