Laboratory and industrial cultures of Escherichia coli employ media containing glucose which is mainly transported and phosphorylated by the phosphotransferase system (PTS). In these strains, 50% of the phosphoenolpyruvate (PEP), which results from the catabolism of transported glucose, is used as a phosphate donor for its phosphorylation and translocation by the PTS. This characteristic of the PTS limits the production of industrial biocommodities that have PEP as a precursor. Furthermore, when E. coli is exposed to carbohydrate mixtures, the PTS prevents expression of catabolic and non-PTS transport genes by carbon catabolite repression and inducer exclusion. In this contribution, we discuss the main strategies developed to overcome these potentially limiting effects in production strains. These strategies include adaptive laboratory evolution selection of PTS^- Glc⁺ mutants, followed by the generation of strains that recover their ability to grow with glucose as a carbon source while allowing the simultaneous consumption of more than one carbon source. We discuss the benefits of using alternative glucose transport systems and describe the application of these strategies to E. coli strains with specific genetic modifications in target pathways. These efforts have resulted in significant improvements in the production of diverse biocommodities, including aromatic metabolites, biofuels and organic acids.

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Aromatic metabolism comprises the shikimic acid (SA) and the aminoshikimic acid (ASA) pathways. The SA pathway is the common route for the biosynthesis of aromatic amino acids and other metabolites in bacteria, higher plants, fungi, and Apicomplexa parasites, but this pathway is absent in mammals. A variant of the SA pathway known as the ASA pathway branches off from the normal pathway in some bacteria, and its final product, 3-amino-5-hydroxybenzoic acid, is the precursor for many aminoglycoside antibiotics such as kanamycin, neomycin, butirosin, and spectinomycin. The SA pathway includes the key intermediate SA, which is the precursor for the chemical synthesis of the drug oseltamivir phosphate, known commercially as Tamiflu ® , an efficient inhibitor of the neuraminidase enzyme of the seasonal influenza viruses types A and B, avian influenza virus H5N1, and human influenza virus H1N1. Meanwhile, the intermediate of the ASA pathway, ASA, is an attractive candidate for use as the core scaffold for the synthesis of combinatorial libraries and is a potential alternative to SA as a precursor for oseltamivir phosphate synthesis. In this review, we discuss the relevance of the key intermediates SA and ASA as scaffold molecules for the synthesis of diverse chemicals. We highlight the current and potential pharmaceutical applications of these molecules and discuss the main strategies for the production of these aromatic compounds from natural sources and the application of metabolic engineering strategies in diverse bacterial strains for production through biotechnological processes.

BackgroundAs a metabolic engineering tool, an adaptive laboratory evolution (ALE) experiment was performed to increase the specific growth rate (µ) in an Escherichia coli strain lacking PTS, originally engineered to increase the availability of intracellular phosphoenolpyruvate and redirect to the aromatic biosynthesis pathway. As result, several evolved strains increased their growth fitness on glucose as the only carbon source. Two of these clones isolated at 120 and 200 h during the experiment, increased their μ by 338 and 373 %, respectively, compared to the predecessor PB11 strain. The genome sequence and analysis of the genetic changes of these two strains (PB12 and PB13) allowed for the identification of a novel strategy to enhance carbon utilization to overcome the absence of the major glucose transport system.ResultsGenome sequencing data of evolved strains revealed the deletion of chromosomal region of 10,328 pb and two punctual non-synonymous mutations in the dhaM and glpT genes, which occurred prior to their divergence during the early stages of the evolutionary process. Deleted genes related to increased fitness in the evolved strains are rppH, aas, lplT and galR. Furthermore, the loss of mutH, which was also lost during the deletion event, caused a 200-fold increase in the mutation rate.ConclusionsDuring the ALE experiment, both PB12 and PB13 strains lost the galR and rppH genes, allowing the utilization of an alternative glucose transport system and allowed enhanced mRNA half-life of many genes involved in the glycolytic pathway resulting in an increment in the μ of these derivatives. Finally, we demonstrated the deletion of the aas-lplT operon, which codes for the main components of the phosphatidylethanolamine turnover metabolism increased the further fitness and glucose uptake in these evolved strains by stimulating the phospholipid degradation pathway. This is an alternative mechanism to its regeneration from 2-acyl-glycerophosphoethanolamine, whose utilization improved carbon metabolism likely by the elimination of a futile cycle under certain metabolic conditions. The origin and widespread occurrence of a mutated population during the ALE indicates a strong stress condition present in strains lacking PTS and the plasticity of this bacterium that allows it to overcome hostile conditions.Electronic supplementary materialThe online version of this article (doi:10.1186/s12934-015-0382-6) contains supplementary material, which is available to authorized users.