Benzyl derivative facilitation of transcription in Escherichia coli at the ara and lac operon promoters: metabolite gene regulation (MGR)

Identification of four structural genes and two putative promoters necessary for utilization of naphthalene, phenanthrene, fluoranthene by Sphingomonas paucimobilis var. EPA505

Sphingomonas paucimobilis var. EPA505 utilizes fluoranthene (FLA), naphthalene (NAP), and phenanthrene (PHE) as sole carbon sources for energy and growth. A genetic library of EPA505 was constructed using mini-Tn5 promoter reporter genes encoding for tetracycline resistance (tc(p-)) or luminescence (luxAB(p-)). Out of 2250 Tn5 mutants, ten were deficient in utilization of FLA, NAP, and/or PHE as sole carbon sources. Three classes of Tn5 mutants were defined: classI (nap(-)phe(-)fla(-)), classII (nap(-)phe(-)), and classIII (fla(-)). Four of five mutants in classI did not express dioxygenase function, whereas one classI mutant and all classII and classIII mutants retained dioxygenase activity. In Tn5 tc(p-) classI mutants 200 and 394 (dioxygenase negative) and classII mutant 132 (dioxygenase positive), promoter reporter was expressed when induced with FLA, NAP, PHE, other polycyclic aromatic hydrocarbons (PAHs), and several proposed PAH-derived catabolites. The Tn5 tc(p-) derived classIII mutant 104 was induced only with PAHs and not with PAH-derived catabolites. DNA sequence analysis of cloned regions of classI mutant 200 revealed that Tn5 inserted into a gene that shared (96%) DNA sequence homology with 2,3-dihydroxybiphenyl 1,2-dioxygenase that is designated pbhA. Nucleotide sequences downstream of pbhA shared (84%) homology to a Rieske-type ferredoxin subunit gene of a multicomponent dioxygenase designated pbhB. The Tn5 tc(p-) in classII mutant 132 occurred within sequences that shared (74%) homology with a trans-o-hydroxybenzylidene-pyruvate hydratase-aldolase gene (pbhC). Sequence analysis of the region proximal to this gene revealed a putative promoter that contained a binding site for a LysR transcriptional activator. In classIII mutant 104, the Tn5 tc(p-) resided within a region that shared 94% nucleotide homology to that of a pyruvate phosphate dikinase gene known to be involved in cellular uptake of glucose. The FLA-specific catabolic gene disrupted in mutant 104 was designated phbD. Functional and sequence analyses of promoter probe mutants allowed identification of four genes necessary for the utilization of PAHs that are controlled by at least two promoters that are affected by a wide range of aromatic compounds.

Induction of Escherichia coli hydroperoxidase I by acetate and other weak acids

Escherichia coli produces two independently regulated hydroperoxidases (catalases) that protect the cell from toxic concentrations of hydrogen peroxide. Hydroperoxidase I (HPI) is induced by hydrogen peroxide in an OxyR-dependent manner, while hydroperoxidase II (HPII) synthesis is regulated by an alternative sigma factor called RpoS (KatF). The activities of both hydroperoxidases increase as exponentially growing cells enter stationary phase. In this study, we examined the growth phase-dependent expression of HPI. Treatment of early-exponential-phase cells with spent culture supernatant resulted in induction of HPI synthesis. Extracellular levels of hydrogen peroxide, accumulating in the culture supernatant during late exponential phase, were found to be lower than the concentrations normally required to induce OxyR-dependent synthesis of HPI. This finding suggested that factors other than hydrogen peroxide may play a role in HPI expression. Weak acids such as acetate, which accumulate in culture supernatant and have been implicated in the regulation of HPII, caused a sixfold increase in HPI expression. Increases in HPI synthesis, mediated by weak acids and spent culture fluid supernatant, could be prevented by chloramphenicol, indicating that de novo protein synthesis is required for induction. Expression studies using a plasmid-borne lacZ transcriptional fusion to katG, the structural gene for HPI, indicated that growth phase-dependent regulation of HPI occurs primarily at the level of transcription and is dependent on RpoS. These results suggest that there may be a common regulatory mechanism of HPI and HPII expression in addition to previously described independent control mechanisms.

pH dependence and gene structure of inaA in Escherichia coli

The weak-acid-inducible locus inaA in Escherichia coli was mapped to 48.6 min by P1 cotransduction of inaA Mud lac fusions and linked Tn10 insertions. The inaA1::lac fusion tested negative for phenotypes characteristic of mutations in the nearby locus ubiG. Sequence analysis of a fragment amplified by polymerase chain reaction located the inaA1::lac fusion joint within an open reading frame 311 nucleotides downstream of nrdB, transcribed in the opposite direction, encoding a 168-amino-acid polypeptide. Constitutive mutant strains identified on lactose MacConkey revealed a novel regulatory locus unlinked to inaA, which mapped at 34 min (designated inaR). Expression of inaA1::lac increased slightly with external acidification; the presence of benzoate, a membrane-permeant weak acid, greatly increased the acid effect. The expression at various combinations of benzoate and external pH correlated with the decrease in intracellular pH. The uncouplers salicylate and dinitrophenol also caused acid-dependent induction of inaA, but substantial induction was seen at external pH values higher than the internal pH; this effect cannot be caused by internal acidification. Nondissociating analogs of benzoate and salicylate, benzyl alcohol and salicyl alcohol, did not induce inaA. Expression of inaA was inversely related to growth temperature over the range of 30 to 45 degrees C. The inaA1::lac fusion was transferred to a strain defective for K+ uptake (kdpABC trkA trkD) in which pH homeostasis was shown to depend on the external K+ concentration. In this construct, inaA1::lac retained pH-dependent induction by benzoate but was not induced at low K+ concentrations. Induction of inaA appears to involve several factors in addition to internal pH. inaR may be related to the nearby locus marA/soxQ, which is inducible by acidic benzyl derivatives.

Potentiation by salicylate and salicyl alcohol of cadmium toxicity and accumulation in Escherichia coli

The toxicity of Cd2+ in Escherichia coli K-12 was potentiated by salicylate and several related compounds. The efficiency of plating on Luria broth plates was reduced by more than 10(5)-fold when 10 mM salicylate and 200 microM CdCl2 were present simultaneously but was unaffected when either compound was present by itself. Synergistic effects were found at pH 7.4 with certain other weak acids (acetyl salicylate [aspirin], benzoate, and cinnamate) and with a nonacidic salicylate analog, salicyl alcohol, but not with acetate or p-hydroxy benzoate. Thus, the synergism with Cd2+ is determined by the structure of the compounds and not merely by their acidity. The kinetics of 109Cd2+ uptake by cells grown and assayed in broth indicated the presence of two uptake systems with Kms of 1 and 52 microM Cd2+ and Vmaxs of 0.059 and 1.5 mumol of Cd2+ per min per g of cells, respectively. The kinetics of uptake for cells grown and assayed with 20 mM salicyl alcohol showed 2.5-fold increases in the Vmaxs of both systems but no change in the Kms. Salicylate-grown cells also exhibited increased rates of 109Cd2+ uptake by both systems. Thus, enhanced uptake of Cd2+ may be responsible for the potentiation of Cd2+ toxicity by salicylate and salicyl alcohol.

A family of positive regulators related to the Pseudomonas putida TOL plasmid XylS and the Escherichia coli AraC activators

The XylS family consists of a least 8 different transcriptional regulators. Six of these proteins are positive regulators for the catabolism of carbon sources (benzoate and sugars) in Escherichia coli, Pseudomonas putida and Erwinia carotovora, and two of them are involved in pathogenesis in Escherichia coli and Yersinia enterocolitica. Based on protein alignments, the members of this family exhibit a long stretch of homology at the C-terminal end. The regulators involved in the catabolism of carbon sources stimulate transcription from their respectively regulated promoters only in the presence of effectors. In two of the regulators, mutations at the non-homologous N-terminus alter affinity and specificity for effectors while mutations at the conserved C-terminus part decrease activation of transcription from their corresponding regulated promoters. It is thus probable that the variable N-terminus end in this family of regulators contains the motif involved in effector recognition, while the C-terminal end is involved in DNA-binding. These proteins seem to be related by common ancestry and may act through similar mechanisms of positive regulation effected through similar folding patterns.

Signal-regulator interactions. Genetic analysis of the effector binding site of xylS, the benzoate-activated positive regulator of Pseudomonas TOL plasmid meta-cleavage pathway operon

This study reports a genetic analysis of the interactions between a positive regulator of gene expression and its effector molecules. Transcription of the TOL plasmid meta-cleavage pathway operon is specifically stimulated by the XylS protein positive regulator either through activation of this regulator by benzoate effectors or through its hyperproduction. One xylS mutant that exhibits constitutive expression of the operon promoter has been characterized, together with six mutants encoding altered XylS proteins that recognize as effectors benzoate analogues that are non-effectors for the XylS wild-type protein. The changes in two mutant regulators are located at the N-terminal end of the protein, within a putative beta-pleated domain. These mutant proteins exhibit a markedly increased affinity for normal benzoate effectors, with K's values fivefold to 60-fold lower than those of the wild-type XylS protein. They are additionally activated by new effectors having certain substituents at position 2, 3 and 4 of the aromatic ring. Two other mutant proteins recognize new effectors having substituents at position 4 and 5 of the aromatic ring, and contain mutations at their C-terminal end within a putative alpha-helix-rich domain. Three other mutations, one of which leads to constitutive expression from Pm, each result in an amino acid change in the central region of the regulator. These findings suggest but do not prove that the effector binding pocket of the XylS protein may be composed of two or more non-contiguous segments of its primary structure. The XylS protein exhibits homology with the AraC protein of Escherichia coli, a protein that stimulates transcription from ara promoters when it is activated by arabinose or benzoate. Mutations influencing effector activation of the XylS protein characterized in this study are all located in regions exhibiting a high degree of homology with the corresponding aligned sequence of AraC protein.

The effects of anthranilic acid on gene expression

1. A Drosophila pseudoobscura amylase gene cloned in Escherichia coli is expressed at high levels. The expression of this gene is repressed when glucose (0.5% final concentration) is added to a starch minimal medium culture of E. coli cells containing the amylase plasmid. 2. Addition of anthranilic acid (5 and 7 mM final exogenous concentration) to catabolite repressed cells mimics the action of adenosine 3'5' cyclic monophosphate (cAMP) by depressing the expression of the amylase. 3. The results suggest that anthranilic acid acts either indirectly, possibly through the glucose transport system, or directly, by way of an intercalative model of initiation, to alter the levels of transcription.

Inhibition of imidazole-induced tyrosinase activity by estradiol and estriol in cultured B16/C3 melanoma cells

The effect of estrogens on tyrosinase (EC 1.14.18.1) activity was studied in B16/C3 melanoma cultures. Estradiol, estriol, and other related steroids failed to influence tyrosinase activity when added to the medium of proliferating cultures. Imidazole (10 mM), on the other hand, induced the activity of that enzyme 3-fold, as reported previously. Estradiol and estriol blocked imidazole induction, however, unlike the other estrogenic compounds. The blockade occurred within 15 min of hormone addition and was reversible. Dose-response studies revealed that the maximal estradiol effect occurred at 0.75 nM and the half-maximal effect occurred at 0.5 nM. Estriol was more potent, with the maximal blockade occurring at approximately 0.5 nM and half-maximal effect at 0.25 nM. The induction of tyrosinase by imidazole and the blockade of this induction by estradiol and estriol could not be demonstrated in broken cell preparations, suggesting that direct enzyme activation-inactivation was not involved. Studies utilizing inhibitors of protein and RNA synthesis suggest that this effect is mediated at a pre-translational level and is independent of mRNA destabilization.