Quantitative aspects of active transport by the lactose transport system of Escherichia coli

Maloney, Peter C.; Wilson, Thérèse

doi:10.1016/0005-2736(73)90225-3

Cited by 49 publications

(33 citation statements)

References 9 publications

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“…Table 4 shows that lactose accumulated at less than 10% maximal lac operon expression induced the mel operon nearly as well as when the lac operon was fully induced. A similar observation has been made with TMG; cells that express the lac operon at a fractional level accumulate TMG to a steady-state concentration close to that of cells with a fully induced lac operon (33). Another interesting observation is that, although the mel operon was significantly induced by TMG accumulated by the Lac permease, the induction was not maintained by TMG in the absence of Lac permease, despite the fact that Mel permease also transports TMG (11).…”

Section: Discussionsupporting

confidence: 69%

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

Narang

Oehler

2017

J Bacteriol

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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

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Section: Discussionsupporting

confidence: 69%

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

Narang

Oehler

2017

J Bacteriol

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“…Substrate entry measured in −PE þ PC cells was via LacY as evidenced by lack of TMG and lactose uptake by AL95∕pAC-PCS lp -Sp-Gm without a plasmid copy of LacY. Comparative rates of uphill transport are difficult to assess especially with multiple copies of lacY because 10% induction of single copy lacY results in 50% of the uphill transport rate after full induction of single copy lacY (24). However, the rate of downhill substrate transport was significantly lower than that observed in þPE cells (Fig.…”

Section: Resultsmentioning

confidence: 99%

Plasticity of lipid-protein interactions in the function and topogenesis of the membrane protein lactose permease from Escherichia coli

Bogdanov¹,

Heacock²,

Guan

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

115

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Phosphatidylcholine (PC) has been widely used in place of naturally occurring phosphatidylethanolamine (PE) in reconstitution of bacterial membrane proteins. However, PC does not support native structure or function for several reconstituted transport proteins. Lactose permease (LacY) of Escherichia coli, when reconstituted in E. coli phospholipids, exhibits energy-dependent uphill and energy-independent downhill transport function and proper conformation of periplasmic domain P7, which is tightly linked to uphill transport function. LacY expressed in cells lacking PE and containing only anionic phospholipids exhibits only downhill transport and lacks native P7 conformation. Reconstitution of LacY in the presence of E. coli-derived PE, but not dioleoyl-PC, results in uphill transport. We now show that LacY exhibits uphill transport and native conformation of P7 when expressed in a mutant of E. coli in which PC completely replaces PE even though the structure is not completely native. E. coli-derived PC and synthetic PC species containing at least one saturated fatty acid also support the native conformation of P7 dependent on the presence of anionic phospholipids. Our results demonstrate that the different effects of PE and PC species on LacY structure and function cannot be explained by differences in the direct interaction of the lipid head groups with specific amino acid residues alone but are due to more complex effects of the physical and chemical properties of the lipid environment on protein structure. This conclusion is supported by the effect of different lipids on the proper folding of domain P7, which indirectly influences uphill transport function.

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“…(25) φ s See Eq. (26) δ m0 310/K −1 x (Maloney and Wilson, 1973) In view of (24), we can assume that…”

Section: The Effect Of Dilutionmentioning

confidence: 99%

“…Now, it may be permissible to neglect the diffusive influx in induced cells because such cells contain such large permease levels that the inducer is imported almost entirely by the permease. However, non-induced cells contain only ∼0.1% of the permease in induced cells (Maloney and Wilson, 1973). In such cells, it seems unlikely that the diffusive influx is negligible.…”

Section: Introductionmentioning

confidence: 98%

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The diffusive influx and carrier efflux have a strong effect on the bistability of the lac operon in Escherichia coli

Noel

Pilyugin

Narang

2009

Journal of Theoretical Biology

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In the presence of gratuitous inducers, the lac operon of Escherichia coli exhibits bistability. Most models in the literature assume that the inducer enters the cell via the carrier (permease), and exits by a diffusion-like process. The diffusive influx and carrier efflux are neglected. However, analysis of the data shows that in noninduced cells, the diffusive influx is comparable to the carrier influx, and in induced cells, the carrier efflux is 7 times the diffusive efflux. Since bistability entails the coexistence of steady states corresponding to both non-induced and induced cells, neither one of these fluxes can be ignored. Here, we formulate a model accounting for both fluxes. We show that: (a) The thresholds of bistability are profoundly affected by both fluxes. The diffusive influx reduces the on threshold by enhancing inducer accumulation in non-induced cells. The carrier efflux increases the off threshold by decreasing inducer accumulation in induced cells. (b) Simulations of the model with experimentally measured parameter values are in good agreement with the data for IPTG. However, there are discrepancies with respect to the data for TMG. They are most likely due to two features missing from the model, namely, the variation of the inducer exclusion effect and the specific growth rate with the lactose enzyme levels. (c) The steady states and thresholds obtained in the presence of both fluxes are well approximated by simple analytical expressions. These expressions provide a rigorous framework for the preliminary design of the lac genetic switch in synthetic biology.

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Quantitative aspects of active transport by the lactose transport system of Escherichia coli

Cited by 49 publications

References 9 publications

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

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

Plasticity of lipid-protein interactions in the function and topogenesis of the membrane protein lactose permease from Escherichia coli

The diffusive influx and carrier efflux have a strong effect on the bistability of the lac operon in Escherichia coli

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