Glucose transport in Escherichia coli mutant strains with defects in sugar transport systems

Abstract

In Escherichia coli, several systems are known to transport glucose into the cytoplasm. The main glucose uptake system under batch conditions is the glucose phosphoenolpyruvate:carbohydrate phosphotransferase system (glucose PTS), but the mannose PTS and the galactose and maltose transporters also can translocate glucose. Mutant strains which lack the enzyme IIBC (EIIBC) protein of the glucose PTS have been investigated previously because their lower rate of acetate formation offers advantages in industrial applications. Nevertheless, a systematic study to analyze the impact of the different glucose uptake systems has not been undertaken. Specifically, how the bacteria cope with the deletion of the major glucose uptake system and which alternative transporters react to compensate for this deficit have not been studied in detail. Therefore, a series of mutant strains were analyzed in aerobic and anaerobic batch cultures, as well as glucose-limited continuous cultivations. Deletion of EIIBC disturbs glucose transport severely in batch cultures; cyclic AMP (cAMP)-cAMP receptor protein (CRP) levels rise, and induction of the mgl operon occurs. Nevertheless, Mgl activity is not essential for growth of these mutants, since deletion of this transporter did not affect the growth rate; the activities of the remaining transporters seem to be sufficient. Under conditions of glucose limitation, mgl is upregulated 23-fold compared to levels for growth under glucose excess. Despite the strong induction of mgl upon glucose limitation, deletion of this transport system did not lead to further changes. Although the galactose transporters are often regarded as important for glucose uptake at micromolar concentrations, the glucose as well as mannose PTS might be sufficient for growth at this relatively low dilution rate.

Fig 1

Main regulation of uptake of glucose via the phosphoenolpyruvate phosphotransferase system.

Fig 2

Expression of genes coding for carbohydrate uptake systems in E. coli MG1655 grown under anaerobic versus aerobic conditions with glucose excess, normalized to anaerobic conditions. ptsG encodes EIIBC^Glc; mglB, encodes a subunit of the galactose ABC transporter MglBAC; galP encodes galactose permease; malE encodes a subunit of the maltose ABC transporter MalEFG; manX encodes EIIAB^Man. Average data from 4 independent cultivations per strain and condition are given. Gene expression was normalized to the expression level under anaerobic conditions. The horizontal lines mark the significance levels; only changes above the solid line or below the dotted one were regarded as significant.

Fig 3

Growth rate of E. coli MG1655 and isogenic mutant strains in glucose minimal medium under anaerobic and aerobic conditions. The mutant strains bear one or more deletions of the galactose transport system (MglBAC, GalP), maltose transporter (MalEFG), mannose PTS (ManXYZ), or glucose PTS (PtsG).

Fig 4

(a and b) Aerobic glucose consumption rate (a) or aerobic by-product yield (b) of E. coli MG1655 and isogenic mutant strains. (c) Anaerobic and aerobic biomass and metabolite yield of MG1655 in glucose minimal medium. The mutant strains bear one or more deletions in the galactose transport system (Mgl and GalP), a component of the maltose transporter (MalE), the mannose PTS (ManX), or the glucose PTS (PtsG).

Fig 5

Expression of transport systems in E. coli MG1655 and isogenic mutant strains in glucose minimal medium under aerobic conditions (a) or anaerobic conditions (b). The mutant strains bear deletions of the glucose PTS (PtsG), the galactose transport system (MglBAC and GalP), a component of the maltose transporter (MalEFG), or the mannose PTS (ManXYZ). Average data from 4 independent cultivations per strain and condition are shown. Gene expression was normalized to the expression level of the wild type. The horizontal lines mark the significance levels; only changes above the solid line or below the dotted one were regarded as significant.

Fig 6

Expression of several regulators involved in carbon catabolism in E. coli MG1655 and isogenic mutant strains. (a) Expression under aerobic conditions. (b) Expression under anaerobic conditions. The mutant strains bear one or more deletions in the galactose transport system (Mgl and GalP), a component of the maltose transporter (MalE), or the mannose PTS (ManX). Average data from 4 independent cultivations per strain and condition are shown. Gene expression was normalized to the expression level of the wild type. The horizontal lines mark the significance levels; only changes above the solid line or below the dotted one were regarded as significant.

Fig 7

Changes in gene expression rates for E. coli MG1655 and isogenic mutant strains grown under glucose limitation versus growth with glucose excess. The mutant strains bear one or more deletions in the galactose transport system (Mgl and GalP), a component of the maltose transporter (MalE), or the mannose PTS (ManX). Average data from 2 independent cultivations per strain and condition are shown. Gene expression was normalized to the expression level under glucose-excess conditions for each strain. The horizontal lines mark the significance levels; only changes above the solid line or below the dotted one were regarded as significant.

Fig 8

Changes in gene expression rates for E. coli MG1655 and isogenic mutant strains grown under glucose limitation versus growth with glucose excess. The mutant strains bear one or more deletions in the galactose transport system (Mgl and GalP), a component of the maltose transporter (MalE), or the mannose PTS (ManX). Average data from 2 independent cultivations per strain and condition are shown. Gene expression was normalized to the expression level under glucose-excess conditions for each strain. The horizontal lines mark the significance levels; only changes above the solid line or below the dotted one were regarded as significant.