Mutations in phoB, the positive gene activator of the pho regulon in Escherichia coli, affect the carbohydrate phenotype on MacConkey indicator plates

Exposure to Glycolytic Carbon Sources Reveals a Novel Layer of Regulation for the MalT Regulon

Bacteria adapt to changing environments by means of tightly coordinated regulatory circuits. The use of synthetic lethality, a genetic phenomenon in which the combination of two nonlethal mutations causes cell death, facilitates identification and study of such circuitry. In this study, we show that the E. coli ompR malT(con) double mutant exhibits a synthetic lethal phenotype that is environmentally conditional. MalT(con), the constitutively active form of the maltose system regulator MalT, causes elevated expression of the outer membrane porin LamB, which leads to death in the absence of the osmoregulator OmpR. However, the presence and metabolism of glycolytic carbon sources, such as sorbitol, promotes viability and unveils a novel layer of regulation within the complex circuitry that controls maltose transport and metabolism.

Regulation of carbon utilization by sulfur availability in Escherichia coli and Salmonella typhimurium

Different pleiotropic transcriptional regulators are known to function in the coordination of regulons concerned with carbon, nitrogen, sulfur, phosphorus and iron metabolism, but how expression profiles of these different regulons are coordinated with each other is not known. The basis for the effects of cysB mutations on carbon utilization in Escherichia coli and Salmonella typhimurium was examined. cysB mutations affected the utilization of some carbon sources more than others and these effects could be partially, but not completely, reversed by the inclusion of cysteine or djenkolate in the growth medium. Assays of transport systems and enzymes concerned with glucitol and alanine utilization showed that these activities were depressed in cysB mutants relative to isogenic wild-type strains, and cysteine or djenkolate present in the growth media partially restored these activities. Using transcriptional fusions to the fdo (formate dehydrogenase) and gut (glucitol) operons, it was shown that decreased expression resulted from defects at the transcriptional level. Furthermore, the effects of loss of CysB were much less pronounced under conditions of catabolite repression than in the absence of a catabolite-repressing carbon source, and cAMP largely reversed the effect of the loss of CysB. Comparable effects were seen for E. coli lacZ gene expression under the control of its own native promoter, and sulfur limitation in a cysB mutant depressed net cAMP production in a cAMP phosphodiesterase mutant. Adenylate cyclase thus appears to be responsive to sulfur deprivation. These observations may have physiological significance allowing carbon and sulfur regulon coordination during the growth of enteric bacteria in response to nutrient availability.

Glycerol-3-phosphate-mediated repression of malT in Escherichia coli does not require metabolism, depends on enzyme IIAGlc and is mediated by cAMP levels

malT encodes the central activator of the maltose system in Escherichia coli, a gene that is typically under positive control of the cAMP/CAP catabolite repression system. When cells were grown in tryptone broth, the addition of glycerol reduced malT expression two- to threefold. Phosphorylation of glycerol to glycerol-3-phosphate (G3P) was necessary for this repression, but further metabolism to dihydroxyacetone phosphate was not. Mutants lacking adenylate cyclase and harbouring a crp* mutation (synthesizing a cAMP receptor protein that is independent of cAMP) no longer repressed a transcriptional malT-lacZ fusion but still repressed a translational malT-lacZ fusion. Similar results were obtained with a mutant lacking enzyme IIAGlc. For the translational fusion (in a cya crp* genetic background) to be repressed by glycerol, a drop to pH 5 of the growth medium was necessary. Thus, while transcriptional repression by glycerol requires enzyme IIAGlc, cAMP and CAP, pH-mediated translational repression is cAMP independent. Other sugars that are not transported by the phosphotransferase system, most notably D-xylose, showed the same effect as glycerol.

Maltose/maltodextrin system of Escherichia coli: transport, metabolism, and regulation

The maltose system of Escherichia coli offers an unusually rich set of enzymes, transporters, and regulators as objects of study. This system is responsible for the uptake and metabolism of glucose polymers (maltodextrins), which must be a preferred class of nutrients for E. coli in both mammalian hosts and in the environment. Because the metabolism of glucose polymers must be coordinated with both the anabolic and catabolic uses of glucose and glycogen, an intricate set of regulatory mechanisms controls the expression of mal genes, the activity of the maltose transporter, and the activities of the maltose/maltodextrin catabolic enzymes. The ease of isolating many of the mal gene products has contributed greatly to the understanding of the structures and functions of several classes of proteins. Not only was the outer membrane maltoporin, LamB, or the phage lambda receptor, the first virus receptor to be isolated, but also its three-dimensional structure, together with extensive knowledge of functional sites for ligand binding as well as for phage lambda binding, has led to a relatively complete description of this sugar-specific aqueous channel. The periplasmic maltose binding protein (MBP) has been studied with respect to its role in both maltose transport and maltose taxis. Again, the combination of structural and functional information has led to a significant understanding of how this soluble receptor participates in signaling the presence of sugar to the chemosensory apparatus as well as how it participates in sugar transport. The maltose transporter belongs to the ATP binding cassette family, and although its structure is not yet known at atomic resolution, there is some insight into the structures of several functional sites, including those that are involved in interactions with MBP and recognition of substrates and ATP. A particularly astonishing discovery is the direct participation of the transporter in transcriptional control of the mal regulon. The MalT protein activates transcription at all mal promoters. A subset also requires the cyclic AMP receptor protein for transcription. The MalT protein requires maltotriose and ATP as ligands for binding to a dodecanucleotide MalT box that appears in multiple copies upstream of all mal promoters. Recent data indicate that the ATP binding cassette transporter subunit MalK can directly inhibit MalT when the transporter is inactive due to the absence of substrate. Despite this wealth of knowledge, there are still basic issues that require clarification concerning the mechanism of MalT-mediated activation, repression by the transporter, biosynthesis and assembly of the outer membrane and inner membrane transporter proteins, and interrelationships between the mal enzymes and those of glucose and glycogen metabolism.

Effect of growth conditions on expression of the acid phosphatase (cyx-appA) operon and the appY gene, which encodes a transcriptional activator of Escherichia coli

The expression and transcriptional regulation of the Escherichia coli cyx-appA operon and the appY gene have been investigated under different environmental conditions with single-copy transcriptional lacZ fusions. The cyx-appA operon encodes acid phosphatase and a putative cytochrome oxidase. ArcA and AppY activated transcription of the cyx-appA operon during entry into stationary phase and under anaerobic growth conditions. The expression of the cyx-appA operon was affected by the anaerobic energy metabolism. The presence of the electron acceptors nitrate and fumarate repressed the expression of the cyx-appA operon. The nitrate repression was partially dependent on NarL. A high level of expression of the operon was obtained in glucose medium supplemented with formate, in which E. coli obtains energy by fermentation. The formate induction was independent of the fhlA gene product. The results presented in this paper indicate a clear difference in the regulation of the cyx-appA operon and that of the cyd operon, encoding the cytochrome d oxidase complex. The results suggest that cytochrome x oxidase has a function under even more-oxygen-limiting conditions than cytochrome d oxidase. The expression of the appY gene is induced immediately by anaerobiosis, and this anaerobic induction is independent of Fnr, and AppY, but dependent on ArcA. The expression of the appY gene is not affected significantly by the anaerobic energy metabolism, i.e., fermentation versus anaerobic respiration. A model incorporating the anaerobic regulation of the appY gene and the two operons which are controlled by AppY, the hydrogenase 1 (hya) operon and the acid phosphatase (cyx-appA) operon, is presented. The expression of the appY gene is inversely correlated with the growth rate and is induced by phosphate starvation as well as during entry into stationary phase. During oxygen-limiting conditions the stationary-phase induction is partially dependent on ArcA. The alternative sigma factor sigma S has limited influence on the transcription of the appY gene during entry into stationary phase and no effect on the induction by phosphate starvation.

Kinetic analysis by in vivo 31P nuclear magnetic resonance of internal Pi during the uptake of sn-glycerol-3-phosphate by the pho regulon-dependent Ugp system and the glp regulon-dependent GlpT system

When sn-glycerol-3-phosphate (G3P) is taken up exclusively by the pho regulon-dependent Ugp transport system, it can be used as the sole source of Pi but not as the sole source of carbon. We had previously suggested that the inability of G3P to be used as a carbon source under these conditions is due to trans inhibition of G3P uptake by internal Pi derived from the degradation of G3P (P. Brzoska, M. Rimmele, K. Brzostek, and W. Boos, J. Bacteriol. 176:15-20, 1994). Here we report 31P nuclear magnetic resonance measurements of intact cells after exposure to G3P as well as to Pi, using different mutants defective in pst (high-affinity Pi transport), ugp (pho-dependent G3P transport), glpT (glp-dependent G3P transport), and glpD (aerobic G3P dehydrogenase). When G3P was transported by the Ugp system and when metabolism of G3P was allowed (glpD+), Pi accumulated to about 13 to 19 mM. When G3P was taken up by the GlpT system, the preexisting internal Pi pool (whether low or high) did not change. Both systems were inversely controlled by internal Pi. Whereas the Ugp system was inhibited, the GlpT system was stimulated by elevated internal Pi.