The catabolite repressor/activator (Cra) protein of enteric bacteria

Abstract

Bacteria are frequently faced with metabolic questions which, if answered appropriately, can enhance their reproductive and survival capabilities. They must decide how much of a specific nutrient they will take up and metabolize, which of the various equivalent nutrients that may be available they will select, and how they will respond to changes in relative nutrient concentrations as a function of time. They must also coordinate the acquisition of one essential class of nutrients with those of all other such classes. Thus, Escherichia coli will select glucose over lactose, will limit the rate of glucose uptake to correspond to its needs even though it possesses a greater uptake capacity, and will further restrict its carbon uptake in the absence of a required source of nitrogen, phosphorus, or sulfur. We are just now coming to appreciate the molecular details of the processes by which these regulatory interactions are achieved.In the 1970s and 1980s, the molecular details of catabolite repression in E. coli, mediated by cyclic AMP (cAMP) and its cognate receptor, the cAMP receptor protein (CRP), were elucidated (2, 26). It came to be accepted that cAMP provided the principal mechanism of catabolite repression in this enteric bacterium. As key details of the process became known, many investigators became convinced that the essence of the phenomenon had already been solved and that an understanding of this regulatory mechanism would quickly result in elucidation of other essential bacterial regulatory mechanisms. Consequently, interest, research, and funding for bacterial regulatory phenomena of this type began to subside.As early as 1978, evidence had appeared suggesting that cAMP-independent mechanisms of catabolite repression were operative in E. coli (7,15). Similarly, experiments conducted with Bacillus subtilis and other bacteria led to the suggestion that these organisms might possess multiple cAMP-independent mechanisms of catabolite repression (8,25). Moreover, evidence began to accumulate suggesting that the mechanisms of catabolite repression in evolutionarily divergent bacteria are not the same (17,37,38). These facts led to the conclusion that our knowledge of catabolite repression, based on an understanding of the cAMP-CRP-mediated regulatory process in E. coli, represented just the tip of the iceberg (35).The catabolite repressor/activator (Cra) protein of enteric bacteria was initially characterized as the fructose repressor, FruR. Mutants defective in the cra gene (previously designated fruR) exhibited a pleiotropic phenotype, being unable to grow with gluconeogenic substrates as the sole carbon source (5, 13). It became clear that the product of the cra gene controlled the transcriptional expression of numerous genes concerned with carbon and energy metabolism (4,12,14,39). We summarize here the evidence suggesting that the Cra protein, a member of the LacI-GalR family, recognizes an imperfect palindromic DNA sequence to which it binds asymmetrically. If this Cra operator precedes the RNA polym...

Homolactate Fermentation by Metabolically Engineered Escherichia coli Strains

“…Theoretically, the glyoxylate shunt could compensate for a ppc mutation by generating TCA cycle intermediates from acetate. Despite reports of the shunt also being subject to glucose catabolite repression (19,21,22), our NMR results demonstrate that this shunt is at least able to generate succinate from acetate. Why was this succinate unable to support growth in the absence of PEP carboxylase?…”

“…Although PEP carboxykinase (encoded by the pckA gene) and malic enzymes (encoded by maeB and sfcA) also catalyze reversible C-3 carboxylation/ C-4 decarboxylation reactions, these two pathways appear to be responsible for decarboxylation in E. coli rather than for carboxylation (22). Moreover, the pckA, maeB, and sfcA genes are subject to catabolite repression in the presence of glucose (19,21,22). Theoretically, the glyoxylate shunt could compensate for a ppc mutation by generating TCA cycle intermediates from acetate.…”

Genomic Sequencing Reveals Regulatory Mutations and Recombinational Events in the Widely Used MC4100 Lineage ofEscherichia coliK-12

“…First, one of the intentionally introduced deletions contributing to the genotype is now shown to have the effect of disrupting fruR (cra) between codons 82 and 83. FruR regulates multiple genes in central metabolic pathways (28) and so would be expected to affect metabolic regulation. The fruR defect can explain the observed lower expression of the glyoxylate cycle in strain MC4100(MuLac) than in strain MG1655 (26) because it lacks the activation of aceBAK caused by FruR (28).…”

“…FruR regulates multiple genes in central metabolic pathways (28) and so would be expected to affect metabolic regulation. The fruR defect can explain the observed lower expression of the glyoxylate cycle in strain MC4100(MuLac) than in strain MG1655 (26) because it lacks the activation of aceBAK caused by FruR (28). Also documented and confirmed in the genome is the presence of an IS5 element upstream from fnr that differentially affects anaerobic regulation in strain MC4100 (29).…”

Cra-mediated regulation of Escherichia coli adenylate cyclase

“…Expression of the fruBKA operon encoding FruB, FruK (fructose-1-phosphate kinase) and FruA [the fructosespecific Enzyme I1 B'BC (Reizer et al, 1994)] is negatively controlled by a transcription factor originally designated FruR and recently renamed Cra for 'catabolite repressor/activator ' (Saier & Ramseier, 1996 ;. cra mutant strains were first isolated as suppressor mutants that allowed ptsH mutant strains to grow on PTS-sugars (Chin et al, 1987;Kornberg & Prior, 1989).…”