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 ␤-galactosides) and the mel (melibiose) operon (which processes ␣-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, ␤-thiogalactosides like TMG (methyl-␤-D-thiogalactopyranoside) and IPTG (isopropyl-␤-Dthiogalactopyranoside) have not generally been considered to be inducers of the mel operon. The same is true for ␤-galactosides such as lactose [␤-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 ␤-thiogalactosides is observed in the presence of Lac permease, and strong induction by lactose (more than 200-fold) is observed in the absence of ␤-galactosidase. This finding calls for reevaluation of TMG uptake experiments as assays for Lac permease that were performed with mel ϩ strains. IMPORTANCEThe typical textbook picture of bacterial operons is that of standalone 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 lac and mel operons. Here, we show that this set has to be extended to include ␤-galactosides, which have been, until now, considered not to effect the expression of the mel operon. That they can be inducers of the mel operon as well as the lac operon has not been noted in decades of research because of the Escherichia coli genetic background used in previous studies.KEYWORDS beta-galactosides, coactivator, inducer, lac operon, mel operon O perons and regulons are commonly regarded as more or less independent functional units of bacterial metabolism. Relatively few publications deal with regulatory overlaps between different operons of Escherichia coli (e.g., the gal and mgl operons [1]). Another pair of operons with well-known interactions are the lac (2, 3) and mel (4, 5) operons. The mel operon consists of the genes melA (which encodes Mel ␣-galactosidase) and melB (which encodes Mel permease) (6). Their expression is controlled by the AraC-type activator protein MelR, whose gene lies in opposite orientation immediately upstream of the mel promoter (7). Today, the mel operon is mainly known as a model system for permease function (8, 9) and promoter regulation (10). Interest in the mel operon arose initially from the facts that its permease shares the substrate TMG (methyl-␤-D-thiogalactopyranoside) with the lactose (Lac) permease (11) and that the inducer and substrate of the mel ␣-galactosidase, melibiose (6-␣-D-galactopyranosyl-D-glucose), is an inducer of the lac operon, as are several other

Neuromodulators as Interdomain Signaling Molecules Capable of Occupying Effector Binding Sites in Bacterial Transcription Factors

“…If lactose is available, its natural isomer allolactose relieves repression by occupying the DNA-binding site in LacI, thus interfering with its repressor function. Synthetic β-galactosides, including IPTG, as well as some other compounds, such as methyl-β-d-thiogalactopyranoside, can replace allolactose in this function [126]. It is, therefore, likely that neuromodulators that have demonstrated a high affinity and specificity for the LacI ligand binding site (Table 2) may also act as inducers of the lac operon.…”

Microbial approach of epitope tagged MFE-23 single fragment antibodies production

“…Additionally, the use of microbial agents does not produce pollutant either organically or inorganically. 19 The expression of protein is commonly driven by self-replicating plasmid containing robust promoter such as the bacteriophage T7, the synthetic tryptophan operon (trp) promoter, or the E. coli lactose operon (lac). In this study, E. coli was used by following an insertion of the sequences of MFE-23 coding into a bacterial plasmid (pUC119).…”