The maltose/maltodextrin regulon of Escherichia coli consists of 10 genes which encode a binding proteindependent ABC transporter and four enzymes acting on maltodextrins. All mal genes are controlled by MalT, a transcriptional activator that is exclusively activated by maltotriose. By the action of amylomaltase, we prepared uniformly labeled [ 14 C]maltodextrins from maltose up to maltoheptaose with identical specific radioactivities with respect to their glucosyl residues, which made it possible to quantitatively follow the rate of transport for each maltodextrin. Isogenic malQ mutants lacking maltodextrin phosphorylase (MalP) or maltodextrin glucosidase (MalZ) or both were constructed. The resulting in vivo pattern of maltodextrin metabolism was determined by analyzing accumulated [ 14 C]maltodextrins. MalP ؊ MalZ ؉ strains degraded all dextrins to maltose, whereas MalP ؉ MalZ ؊ strains degraded them to maltotriose. The labeled dextrins were used to measure the rate of transport in the absence of cytoplasmic metabolism. Irrespective of the length of the dextrin, the rates of transport at a submicromolar concentration were similar for the maltodextrins when the rate was calculated per glucosyl residue, suggesting a novel mode for substrate translocation. Strains lacking MalQ and maltose transacetylase were tested for their ability to accumulate maltose. At 1.8 nM external maltose, the ratio of internal to external maltose concentration under equilibrium conditions reached 10 6 to 1 but declined at higher external maltose concentrations. The maximal internal level of maltose at increasing external maltose concentrations was around 100 mM. A strain lacking malQ, malP, and malZ as well as glycogen synthesis and in which maltodextrins are not chemically altered could be induced by external maltose as well as by all other maltodextrins, demonstrating the role of transport per se for induction.

Strains of Escherichia coli lacking MalQ (maltodextrin glucanotransferase or amylomaltase) are endogenously induced for the maltose regulon by maltotriose that is derived from the degradation of glycogen (glycogen-dependent endogenous induction). A high level of induction was dependent on the presence of MalP, maltodextrin phosphorylase, while expression was counteracted by MalZ, maltodextrin glucosidase. Glycogenderived endogenous induction was sensitive to high osmolarity. This osmodependence was caused by MalZ. malZ, the gene encoding this enzyme, was found to be induced by high osmolarity even in the absence of MalT, the central regulator of all mal genes. The osmodependent expression of malZ was neither RpoS nor OmpR dependent. In contrast, the malPQ operon, whose expression was also increased at a high osmolarity, was partially dependent on RpoS. In the absence of glycogen, residual endogenous induction of the mal genes that is sensitive to increasing osmolarity can still be observed. This glycogen-independent endogenous induction is not understood, and it is not affected by altering the expression of MalP, MalQ, and MalZ. In particular, its independence from MalZ suggests that the responsible inducer is not maltotriose.The Escherichia coli maltodextrin system has become a paradigm for the understanding of a complex sugar-utilizing system in bacteria (3, 33). The regulon, controlled by MalT, the central activator of the system, consists of 10 coordinately regulated genes that are geared for the utilization of maltose and maltodextrins (for a detailed description of the different aspects of transport, enzymatic activity, and regulation, see the introduction of the accompanying publication [13]).One of the less clear phenomena in maltose regulation is endogenous induction. The degradation of glycogen yields maltodextrins which are channeled back into metabolism by MalQ and MalP (Fig. 1). Among these dextrins is maltotriose. In the absence of MalQ, maltotriose is no longer channeled into metabolism and is therefore able to activate MalT (14). This is the reason why malQ strains appear constitutive (12). We will refer to this type of endogenous induction as glycogenderived endogenous induction. However, even in strains lacking glycogen, the maltose system can be induced internally by growing the cells on carbon sources that yield internal glucose and ␣-glucose-1-phosphate (glucose-1-P) or glucose-6-P. For instance, the metabolism of trehalose is notorious for this production of internal inducer (11,22). It remains unclear whether the active inducer formed under these conditions is in fact maltotriose. Glycogen-independent induction will not be dealt with in this publication.In the absence of MalK, which is the major inhibiting factor of MalT (and which is counteracted by maltotriose) (20, 29), endogenous induction can most easily be recognized since little endogenous inducer is necessary under these conditions. Thus, even cultures grown in glycerol, which is known to cause catabolite repression (15, 16), are c...

2-O-␣-Mannosyl-D-glycerate (MGs)has been recognized as an osmolyte in hyperthermophilic but not mesophilic prokaryotes. We report that MG is taken up and utilized as sole carbon source by Escherichia coli K12, strain MC4100. Uptake is mediated by the P-enolpyruvatedependent phosphotransferase system with the MGinducible HrsA (now called MngA) protein as its specific EIIABC complex. The apparent K m of MG uptake in induced cells was 10 M, and the V max was 0.65 nmol/min/ 10 9 cells. Inverted membrane vesicles harboring plasmid-encoded MngA phosphorylated MG in a P-enolpyruvate-dependent manner. A deletion mutant in mngA was devoid of MG transport but is complemented by a plasmid harboring mngA. Uptake of MG in MC4100 also caused induction of a regulon specifying the uptake and the metabolism of galactarate and glucarate controlled by the CdaR activator. The ybgG gene (now called mngB) the gene immediately downstream of mngA encodes a protein with ␣-mannosidase activity. farR, the gene upstream of mngA (now called mngR) had previously been characterized as a fatty acyl-responsive regulator; however, deletion of mngR resulted in the up-regulation of only two genes, mngA and mngB. The mngR deletion caused constitutive MG transport that became MG-inducible after transformation with plasmid expressed mngR. Thus, MngR is the regulator (repressor) of the MG transport/metabolism system. Thus, the mngR mngA mngB gene cluster encodes an MG utilizing system.

MalT is the central transcriptional activator of all mal genes in Escherichia coli. Its activity is controlled by the inducer maltotriose. It can be inhibited by the interaction with certain proteins, and its expression can be controlled. We report here a novel aspect of mal gene regulation: the effect of cytoplasmic glucose and glucokinase (Glk) on the activity and the expression of MalT. Amylomaltase (MalQ) is essential for the metabolism of maltose. It forms maltodextrins and glucose from maltose or maltodextrins. We found that glucose above a concentration of 0.1 mM blocked the activity of the enzyme. malQ mutants when grown in the absence of maltodextrins are endogenously induced by maltotriose that is derived from the degradation of glycogen. Therefore, the fact that glk malQ ؉ mutants showed elevated mal gene expression finds its explanation in the reduced ability to remove glucose from MalQ-catalyzed maltodextrin formation and is caused by a metabolically induced MalQ ؊ phenotype. However, even in mutants lacking glycogen, Glk controls endogenous induction. We found that overexpressed Glk due to its structural similarity with Mlc, the repressor of malT, binds to the glucose transporter (PtsG), releasing Mlc and thus increasing malT repression. In addition, even in mutants lacking Mlc (and glycogen), the overexpression of glk leads to a reduction in mal gene expression. We interpret this repression by a direct interaction of Glk with MalT concomitant with MalT inhibition. This repression was dependent on the presence of either maltodextrin phosphorylase or amylomaltase and led to the inactivation of MalT.The Escherichia coli maltose system (4, 52) is geared for the efficient utilization of maltose and maltodextrins. Ten mal genes encode proteins found in all compartments of the cell. The lambda receptor in the outer membrane (43,49) facilitates the diffusion of maltodextrins into the periplasmic space, where they are taken up into the cytoplasm via a bindingprotein-dependent ABC transporter (32, 55). There are two main enzymes catalyzing the degradation of maltose and maltodextrins to glucose and ␣-glucose-1-phosphate. Amylomaltase (MalQ) (29), a maltodextrin glucanotransferase (41, 59), forms from any maltodextrin, including maltose, larger maltodextrins, and glucose (16,34,60). Maltotetraose and longer maltodextrins are substrates of the maltodextrin phosphorylase (MalP) (53, 58), yielding by phosphorolysis ␣-glucose-1-phosphate and smaller maltodextrins. Two other enzymes are a periplasmic amylase (MalS) (20, 51) and a cytoplasmic maltodextrin glucosidase (MalZ) that are not essential for maltose or maltodextrin utilization (44,51,57). While MalS produces preferentially maltohexaose from longer maltodextrins in the periplasm, MalZ degrades longer maltodextrins by cleaving glucose from the reducing end of the dextrins in the cytoplasm. The smallest substrate of MalZ is maltotriose, producing maltose and glucose. All mal genes are under the positive control of MalT (45), which in turn is activated by the...