Dataset Information

Bistability and Nonmonotonic Induction of the lac Operon in the Natural Lactose Uptake System.

ABSTRACT: The Escherichia coli lac operon is regulated by a positive feedback loop whose potential to generate an all-or-none response in single cells has been a paradigm for bistable gene expression. However, so far bistable lac induction has only been observed using gratuitous inducers, raising the question about the biological relevance of bistable lac induction in the natural setting with lactose as the inducer. In fact, the existing experimental evidence points to a graded rather than an all-or-none response in the natural lactose uptake system. In contrast, predictions based on computational models of the lactose uptake pathway remain controversial. Although some argue in favor of bistability, others argue against it. Here, we reinvestigate lac operon expression in single cells using a combined experimental/modeling approach. To this end, we parameterize a well-supported mathematical model using transient measurements of LacZ activity upon induction with different amounts of lactose. The resulting model predicts a monostable induction curve for the wild-type system, but indicates that overexpression of the LacI repressor would drive the system into the bistable regime. Both predictions were confirmed experimentally supporting the view that the wild-type lac induction circuit generates a graded response rather than bistability. More interestingly, we find that the lac induction curve exhibits a pronounced maximum at intermediate lactose concentrations. Supported by our data, a model-based analysis suggests that the nonmonotonic response results from saturation of the LacI repressor at low inducer concentrations and dilution of Lac enzymes due to an increased growth rate beyond the saturation point. We speculate that the observed maximum in the lac expression level helps to save cellular resources by limiting Lac enzyme expression at high inducer concentrations.

SUBMITTER: Zander D

PROVIDER: S-EPMC5425413 | biostudies-literature | 2017 May

REPOSITORIES: biostudies-literature

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Publications

Bistability and Nonmonotonic Induction of the lac Operon in the Natural Lactose Uptake System.

Zander Dominique D Samaga Daniel D Straube Ronny R Bettenbrock Katja K

Biophysical journal 20170501 9

The Escherichia coli lac operon is regulated by a positive feedback loop whose potential to generate an all-or-none response in single cells has been a paradigm for bistable gene expression. However, so far bistable lac induction has only been observed using gratuitous inducers, raising the question about the biological relevance of bistable lac induction in the natural setting with lactose as the inducer. In fact, the existing experimental evidence points to a graded rather than an all-or-none ...[more]

PMID: 28494968

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Project description:The role of stochasticity on gene expression is widely discussed. Both potential advantages and disadvantages have been revealed. In some systems, noise in gene expression has been quantified, in among others the lac operon of Escherichia coli. Whether stochastic gene expression in this system is detrimental or beneficial for the cells is, however, still unclear. We are interested in the effects of stochasticity from an evolutionary point of view. We study this question in the lac operon, taking a computational approach: using a detailed, quantitative, spatial model, we evolve through a mutation-selection process the shape of the promoter function and therewith the effective amount of stochasticity. We find that noise values for lactose, the natural inducer, are much lower than for artificial, nonmetabolizable inducers, because these artificial inducers experience a stronger positive feedback. In the evolved promoter functions, noise due to stochasticity in gene expression, when induced by lactose, only plays a very minor role in short-term physiological adaptation, because other sources of population heterogeneity dominate. Finally, promoter functions evolved in the stochastic model evolve to higher repressed transcription rates than those evolved in a deterministic version of the model. This causes these promoter functions to experience less stochasticity in gene expression. We show that a high repression rate and hence high stochasticity increases the delay in lactose uptake in a variable environment. We conclude that the lac operon evolved such that the impact of stochastic gene expression is minor in its natural environment, but happens to respond with much stronger stochasticity when confronted with artificial inducers. In this particular system, we have shown that stochasticity is detrimental. Moreover, we demonstrate that in silico evolution in a quantitative model, by mutating the parameters of interest, is a promising way to unravel the functional properties of biological systems.

Project description:1. Several lac diploid strains of Escherichia coli were constructed and tested to discover whether mutations in the lac promoter alleviate catabolite repression. 2. In each of these diploids the chromosome carries one of the promoter mutations, L8, L29 or L1; so that the rate of synthesis of the enzymes of the lac operon is only 2-6% of the fully induced wild-type. Each diploid harbours the episome F'lacM15 that specifies the synthesis of thiogalactoside transacetylase under the control of intact regulator, promoter and operator regions, but has a deletion in the structural gene for beta-galactosidase. In each diploid more than 90% of the thiogalactoside transacetylase is synthesized from the episome, and 100% of the beta-galactosidase is synthesized from the chromosome, and comparison of the extent of catabolite repression that the two enzymes suffered indicated whether the chromosomal promoter mutation relieves catabolite repression. 3. In the strains in which the promoter carries either of the point mutations L8 or L29 the enzymes were equally repressed, suggesting that neither L8 nor L29 affects catabolite repression. 4. In a diploid strain harbouring the same episome but carrying deletion L1 on the chromosome, synthesis of beta-galactosidase suffered much less repression than that of thiogalactoside transacetylase. 5. In a diploid strain in which the chromosome carries L1 and also a second mutation that increases the rate of expression of lac to that permitted by L8 or L29, the synthesis of beta-galactosidase again suffered much less repression than the synthesis of thiogalactoside transacetylase. 6. The effect of L1 (which deletes the boundary between the i gene and the lac promoter) is ascribed to its bringing the expression of lac under the control of the promoter of the i gene. 7. Even in strains carrying L1, some catabolite repression persists; this is not due to a trans effect from the episome since it occurs equally in a haploid strain with L1.

Dataset Information

Bistability and Nonmonotonic Induction of the lac Operon in the Natural Lactose Uptake System.

Publications

Bistability and Nonmonotonic Induction of the lac Operon in the Natural Lactose Uptake System.

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