Promoter-like mutants with increased expression of the Escherichia coli uridine phosphorylase structural gene.

From an Escherichia coli K-12 strain lacking adenylate cyclase (cya) and cyclic AMP receptor protein (crp), two mutants were isolated that synthesize uridine phosphorylase constitutively. The mutations differ from one another and also from a wild type in the maximum rate of uridine phosphorylase synthesis. They have constitutive expression of the uridine phosphorylase gene (udp) in the presence of repressor protein coded by the cytR regulatory gene and decrease the sensitivity of the udp gene simultaneously with catabolite repression. Both mutations cause a high level of udp expression whether they are in a cya crp or in a cya+ crp+ background. Another mutation (udpP1) isolated previously alters the response of udp gene to the ctyR repressor and produces a higher constitutive level of uridine phosphorylase in a cytR+ than in a cytR background when bacteria are grown in glucose. The synthesis of uridine phosphorylase in this mutant is dependent on an intact cyclic AMP-cyclic AMP receptor protein complex. All mutations studied are cis-acting and extremely closely linked to the udp structural gene, and appear to affect the uridine phosphorylase promoter-operator region. The data obtained are in accordance with a suggestion that the cytR repressor protein normally asserts its function by preventing the positive action of cyclic AMP-cyclic AMP receptor protein complex.     

The cyclic AMP (cAMP)-cAMP receptor protein complex functions both as an activator and as a corepressor at the tsx-p2 promoter of Escherichia coli K-12.

The tsx-p2 promoter is one of at least seven Escherichia coli promoters that are activated by the cyclic AMP (cAMP)-cAMP receptor protein (CRP) complex and negatively regulated by the CytR repressor. DNase I footprinting assays were used to study the interactions of these regulatory proteins with the tsx-p2 promoter region and to characterize tsx-p2 regulatory mutants exhibiting an altered response to CytR. We show that the cAMP-CRP activator complex recognizes two sites in tsx-p2 that are separated by 33 bp: a high-affinity site (CRP-1) overlaps the -35 region, and a low-affinity site (CRP-2) is centered around position -74 bp. The CytR repressor protects a DNA segment that is located between the two CRP sites and partially overlaps the CRP-1 target. In combination, the cAMP-CRP and CytR proteins bind cooperatively to tsx-p2, and the nucleoprotein complex formed covers a region of 78 bp extending from the CRP-2 site close to the -10 region. The inducer for the CytR repressor, cytidine, does not prevent in vitro DNA binding of CytR, but releases the repressor from the nucleoprotein complex and leaves the cAMP-CRP activator bound to its two DNA targets. Thus, cytidine interferes with the cooperative DNA binding of cAMP-CRP and CytR to tsx-p2. We characterized four tsx-p2 mutants exhibiting a reduced response to CytR; three carried mutations in the CRP-2 site, and one carried a mutation in the region between CRP-1 and the -10 sequence. Formation of the cAMP-CRP-CytR DNA nucleoprotein complex in vitro was perturbed in each mutant. These data indicate that the CytR repressor relies on the presence of the cAMP-CRP activator complex to regulate tsx-p2 promoter activity and that the formation of an active repression complex requires the combined interactions of cAMP-CRP and CytR at tsx-p2.     

Effect of mutations for adenylate cyclase (cya) and the cyclic adenosine monophosphate receptor protein (cgp) on the gene expression of nucleoside catabolism in Escherichia coli]

The effect of cya and crp mutations on the expression of the activity of nucleoside catabolizing genes has been studied in Escherichia coli. It is found that cya and crp mutants lose their ability to grow on nucleosides as carbon sources in spite of the preservation of the basal levels of nucleoside catabolizing enzymes, found in cell-free extracts of cya and crp mutants. It is shown that cya and crp mutations completely release the influence of the regulatory gene cytR on the activity of uridine phosphorylase (udp gene) and thymidine phosphorylase (tpp gene). On this ground it is assumed that the cytR gene product acts at the level of promotors of the corresponding structural genes, causing their insensitivity to the positive action of cAMP--CRP complex. The same data concerning the effect of cya and crp mutations on cytR regulation have been reported [8], but these authors favoured the hypothesis that the cytR gene product is a repressor protein, which binds to the specific operator.     

Cloning the uridine phosphorylase (udp) gene of Escherichia coli and its expression in recombinant plasmids

On the basis of Escherichia coli DNA and vectors pBR322, pUC19, hybrid plasmids restoring Udp+ phenotype in the E. coli deletion (delta udp) mutant have been obtained. The udp gene is carried by a 8 kb PstI fragment (on the pUD2) and by a smaller 2.87 kb PstI-SalGI fragment from the PstI fragment (pUD7). The uridine phosphorylase level was 30 times higher in the cells containing hybrid plasmid as compared to the strain with chromosomal location of the udp gene. On the other hand, the measurements of uridine phosphorylase activity in the cytR- and cya- background indicate that expression of the cloned udp gene escapes partially negative control of the CytR repressor and positive control of cAMP--CRP complex. These data suggest that the 2.87 kb PstI--SalGI-fragment contains the intact udp gene which is transcribed from its own promoter. Increase in the activity of beta-galactosidase encoded by udp-lacZ fusion has been observed in the presence of pUD2 or pUD7, which was suggested to be the consequence of titration of CytR repressor molecules in the operator region of the cloned udp.     

Tandem CRP binding sites in the deo operon of Escherichia coli K-12

The locations of DNA binding by the cyclic AMP receptor protein (CRP) in the deo operon of Escherichia coli have been determined by the DNase I footprinting procedure. Two high affinity sites were found around positions -35 and -90, preceding the second deo promoter. In vitro data on induction of gene fusions that join different parts of the deoP -2 regulatory region to the lac genes suggest that: (1) both CRP binding sites are needed for high expression from the deoP -2 region; and (2) negative regulation by the cytR repressor is accomplished by preventing the cAMP-CRP complex from binding to the second target.     

A novel function of the cAMP-CRP complex in Escherichia coli: cAMP-CRP functions as an adaptor for the CytR repressor in the deo operon

Unlike classical bacterial repressors, the CytR repressor of Escherichia coli cannot independently regulate gene expression. Here we show that CytR binding to the deoP2 promoter relies on interaction with the master gene regulatory protein, CRP, and, furthermore, that cAMP-CRP and CytR bind co-operatively to deoP2. Using mutant promoters we show that tandem, properly spaced DNA-bound cAMP-CRP complexes are required for this co-operative binding. These data suggest that CytR forms a bridge between tandem cAMP-CRP complexes, and that cAMP-CRP functions as an adaptor for CytR. The implications of this new version of negative control in E. coli on bacterial gene expression and on combinatorial gene regulation in higher organisms are discussed.