Regulatory region of the metA gene of Escherichia coli K-12

Methionine Limitation Impairs Pathogen Expansion and Biofilm Formation Capacity

Multidrug-resistant bacterial pathogens are becoming increasingly prevalent, and novel strategies to treat bacterial infections caused by these organisms are desperately needed. Bacterial central metabolism is crucial for catabolic processes and provides precursors for anabolic pathways, such as the biosynthesis of essential biomolecules like amino acids or vitamins. However, most essential pathways are not regarded as good targets for antibiotic therapy since their products might be acquired from the environment. This issue raises doubts about the essentiality of such targets during infection. A putative target in bacterial anabolism is the methionine biosynthesis pathway. In contrast to humans, almost all bacteria carry methionine biosynthesis pathways which have often been suggested as putative targets for novel anti-infectives. While the growth of methionine auxotrophic strains can be stimulated by exogenous methionine, the extracellular concentrations required by most bacterial species are unknown. Furthermore, several phenotypic characteristics of methionine auxotrophs are only partly reversed by exogenous methionine. We investigated methionine auxotrophic mutants of Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli (all differing in methionine biosynthesis enzymes) and found that each needed concentrations of exogenous methionine far exceeding that reported for human serum (∼30 µM). Accordingly, these methionine auxotrophs showed a reduced ability to proliferate in human serum. Additionally, S. aureus and P. aeruginosa methionine auxotrophs were significantly impaired in their ability to form and maintain biofilms. Altogether, our data show intrinsic defects of methionine auxotrophs. This result suggests that the pathway should be considered for further studies validating the therapeutic potential of inhibitors.IMPORTANCE New antibiotics that attack novel targets are needed to circumvent widespread resistance to conventional drugs. Bacterial anabolic pathways, such as the enzymes for biosynthesis of the essential amino acid methionine, have been proposed as potential targets. However, the eligibility of enzymes in these pathways as drug targets is unclear because metabolites might be acquired from the environment to overcome inhibition. We investigated the nutritional needs of methionine auxotrophs of the pathogens Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli We found that each auxotrophic strain retained a growth disadvantage at methionine concentrations mimicking those available in vivo and showed that biofilm biomass was strongly influenced by endogenous methionine biosynthesis. Our experiments suggest that inhibition of the methionine biosynthesis pathway has deleterious effects even in the presence of external methionine. Therefore, additional efforts to validate the effects of methionine biosynthesis inhibitors in vivo are warranted.

Effects of DL-methionine supplement on growth performance and amino acid digestion and plasma concentrations in sika deer calves (Cervus nippon)

This experiment was set to investigate the effects of DL-methionine (DL-met) supplement on growth performance and amino acid digestion and plasma concentrations in sika deer calves. Twelve healthy 5-month-old sika deer (29.44 ± 2.86 kg initial bodyweight) were randomly divided into three groups (4/group) and one sika deer per replicate. Levels of Met supplement in three treatments were 0, 1 g/kg and 2 g/kg, respectively. The results showed that the average daily gain for the early 35-day study period tended (P = 0.07) to increase linearly as the Met supplement increased, the feed to gain ratio (F : G) for the early period decreased (linearly and quadratically, P < 0.05), and, in the late 35-day study period, tended (linearly, P = 0.08) to decrease as dietary Met increased. The apparent digestibility of Met increased (linearly and quadratically, P < 0.01) with graded amounts of supplemental Met, and the apparent digestibilities of valine, leucine, isoleucine, phenylalanine, glycine, aspartic and cystine showed a linear increase (P < 0.05). Plasma glycine, proline and isoleucine concentrations increased linearly and quadratically (P < 0.01) with Met supplementation, plasma serine and leucine increased linearly (P < 0.05), and plasma histidine, lysine, arginine and NH3 increased quadratically (P < 0.01 or P < 0.05), with graded amounts of supplemental Met. Met supplementation in the diet improved feed utilisation and amino acid (AA) nutrient digestion, and affected plasma AA concentrations in sika deer calves.

Stabilization of homoserine-O-succinyltransferase (MetA) decreases the frequency of persisters in Escherichia coli under stressful conditions

Bacterial persisters are a small subpopulation of cells that exhibit multi-drug tolerance without genetic changes. Generally, persistence is associated with a dormant state in which the microbial cells are metabolically inactive. The bacterial response to unfavorable environmental conditions (heat, oxidative, acidic stress) induces the accumulation of aggregated proteins and enhances formation of persister cells in Escherichia coli cultures. We have found that methionine supplementation reduced the frequency of persisters at mild (37°C) and elevated (42°C) temperatures, as well as in the presence of acetate. Homoserine-o-succinyltransferase (MetA), the first enzyme in the methionine biosynthetic pathway, is prone to aggregation under many stress conditions, resulting in a methionine limitation in E. coli growth. Overexpression of MetA induced the greatest number of persisters at 42°C, which is correlated to an increased level of aggregated MetA. Substitution of the native metA gene on the E. coli K-12 WE chromosome by a mutant gene encoding the stabilized MetA led to reduction in persisters at the elevated temperature and in the presence of acetate, as well as lower aggregation of the mutated MetA. Decreased persister formation at 42°C was confirmed also in E. coli K-12 W3110 and a fast-growing WErph+ mutant harboring the stabilized MetA. Thus, this is the first study to demonstrate manipulation of persister frequency under stressful conditions by stabilization of a single aggregation-prone protein, MetA.

Potential for development of an Escherichia coli-based biosensor for assessing bioavailable methionine: a review

Methionine is an essential amino acid for animals and is typically considered one of the first limiting amino acids in animal feed formulations. Methionine deficiency or excess in animal diets can lead to sub-optimal animal performance and increased environmental pollution, which necessitates its accurate quantification and proper dosage in animal rations. Animal bioassays are the current industry standard to quantify methionine bioavailability. However, animal-based assays are not only time consuming, but expensive and are becoming more scrutinized by governmental regulations. In addition, a variety of artifacts can hinder the variability and time efficacy of these assays. Microbiological assays, which are based on a microbial response to external supplementation of a particular nutrient such as methionine, appear to be attractive potential alternatives to the already established standards. They are rapid and inexpensive in vitro assays which are characterized with relatively accurate and consistent estimation of digestible methionine in feeds and feed ingredients. The current review discusses the potential to develop Escherichia coli-based microbial biosensors for methionine bioavailability quantification. Methionine biosynthesis and regulation pathways are overviewed in relation to genetic manipulation required for the generation of a respective methionine auxotroph that could be practical for a routine bioassay. A prospective utilization of Escherichia coli methionine biosensor would allow for inexpensive and rapid methionine quantification and ultimately enable timely assessment of nutritional profiles of feedstuffs.

Multilocus sequence typing (MLST) of Escherichia coli O78 strains

Strains of Escherichia coli serotype O78 are associated with many diseases, including invasive infections, in humans and farm animals. The clonal relationship between strains from different hosts is therefore important for assessing the risk of zoonotic infections. Here we propose a multilocus sequence typing scheme for E. coli, based on six housekeeping genes. Preliminary, but significant, results indicate that clonal division in E. coli O78 strains is host independent, and closely related clones reside in different hosts. There was a positive correlation between virulence and clonal origin.

Control of methionine biosynthesis in Escherichia coli by proteolysis

Most bacterial proteins are stable, with half-lives considerably longer than the generation time. In Escherichia coli, the few exceptions are unstable regulatory proteins. The results presented here indicate that the first enzyme in methionine biosynthesis - homoserine trans-succinylase (HTS) - is unstable and subject to energy-dependent proteolysis. The enzyme is stable in triple mutants defective in Lon-, HslVU- and ClpP-dependent proteases. The instability of the protein is determined by the amino-terminal part of the protein, and its removal or substitution by the N-terminal part of beta-galactosidase confers stability. The effect of the amino-terminal segment is not caused by the N-end rule, as substitution of the first amino acid does not affect the stability of the protein. HTS is the first biosynthetic E. coli enzyme shown to have a short half-life and may represent a group of biosynthetic enzymes whose expression is controlled by proteolysis. Alternatively, the proteolytic processing of HTS may be unique to this enzyme and could reflect its central role in regulating bacterial growth, especially at elevated temperatures.

Analysis of the regulatory region of the lysC gene of Escherichia coli

A lysC-lac'Z fusion plasmid was constructed to study the regulatory region of the lysC gene. Analysis by deletion mutations confirmed the existence of an alternative promoter, P2, located upstream of the previously identified promoter, P1. The transcription start site of promoter P2 was located 85 base pairs upstream the transcription start site of promoter P1. Both promoters are regulated by lysine.

Dissecting the molecular details of prokaryotic transcriptional control by surface plasmon resonance: the methionine and arginine repressor proteins

The commercial surface plasmon resonance (SPR) biosensors, BIACORE, have been used to investigate the molecular details of macromolecular interactions at prokaryotic promoter-operators. For the Escherichia coli methionine repressor, MetJ, we have quantitated the interaction of the protein with synthetic and natural operator sites and shown that the SPR response is directly related to the stoichiometry of the complexes being formed. The utility of a continuous flow system has also been exploited to investigate transcription from an immobilised promoter-operator fragment; with transcripts collected and subsequently characterised by RT-PCR. This technique has enabled us to investigate how repressor binding affects (i) the interaction of the RNA polymerase (RNAP) with the promoter and (ii) the ability of RNAP to initiate transcription. Remarkably, the repression complex appears to stabilise binding of RNAP, whilst having the expected effects on the levels of transcripts produced. This may well be a general mechanism allowing rapid transcription initiation to occur as soon as the repression complex dissociates. These techniques have also been used to examine protein-DNA interactions in the E. coli and Bacillus subtilis arginine repressor systems. The repressors are the products of the argR and ahrC genes, respectively. Both proteins form hexamers in rapid equilibrium with smaller subunits believed to be trimers. There are three types of operator in these systems, autoregulatory, biosynthetic and catabolic (B. subtilis only). Sensorgrams show that each protein recognises the three types of immobilised operator differently and that binding is stimulated over 100-fold by the presence of L-arginine.

Molecular characterization and sequence of a methionine biosynthetic locus from Pseudomonas syringae

Two methionine biosynthetic genes in Pseudomonas syringae pv. syringae, metX and metW, were isolated, sequenced, and evaluated for their roles in methionine biosynthesis and bacterial fitness on leaf surfaces. The metXW locus was isolated on a 1.8-kb DNA fragment that was required for both methionine prototrophy and wild-type epiphytic fitness. Sequence analysis identified two consecutive open reading frames (ORFs), and in vitro transcription-translation experiments provided strong evidence that the ORFs encode proteins with the predicted molecular masses of 39 and 22.5 kDa. The predicted amino acid sequence of MetX (39 kDa) showed homology to several known and putative homoserine O-acetyltransferases. This enzyme is the first enzyme in the methionine biosynthetic pathway of fungi, gram-negative bacteria of the genus Leptospira, and several gram-positive bacterial genera. Both metX and metW were required for methionine biosynthesis, and transcription from both genes was not repressed by methionine. MetW (22.5 kDa) did not show significant homology to any known protein, including prokaryotic and eukaryotic methionine biosynthetic enzymes. Several classes of methionine auxotrophs, including metX and metW mutants, exhibit reduced fitness on leaf surfaces, indicating a requirement for methionine prototrophy in wild-type epiphytic fitness. This requirement is enhanced under environmentally stressful conditions, suggesting a role for methionine prototrophy in bacterial stress tolerance.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

An Escherichia coli gene divergently transcribed from a promoter overlapping the metA promoter

The upstream region of the metA gene in Escherichia coli contains two promoters. We have identified by lacZ fusion an additional promoter in this region, and showed that it is transcribed in the opposite orientation from the metA gene. The putative translation product corresponds to a peptide of 147 amino acids-ORF19 by molecular mass. This peptide is probably not essential for growth, as an insertion mutant is viable.

Homoserine O-acetyltransferase, involved in the Leptospira meyeri methionine biosynthetic pathway, is not feedback inhibited

The Leptospira meyeri serovar semaranga metX gene was identified by complementation of an Escherichia coli metA mutant, i.e., devoid of homoserine O-succinyltransferase. However, the MetX protein exhibited a homoserine O-acetyltransferase activity in agreement with its similarity to homoserine O-acetyltransferases. Reverse transcription-PCR analysis demonstrated that metX is the second gene of an operon.

Heat shock-dependent transcriptional activation of the metA gene of Escherichia coli

In Escherichia coli, the growth rate at elevated temperatures is controlled by the availability of endogenous methionine, which is limited because of the temperature sensitivity of the metA gene product, homoserine transsuccinylase (HTS). In order to determine the relationship between this control mechanism and the heat shock response, we estimated the cellular levels of HTS during heat shock by Western (immunoblot) analysis and found an increase following induction by temperature shift and by addition of ethanol or cadmium ions. The elevated level of HTS was a result of transcriptional activation of the metA gene. This activation was heat shock dependent, as it did not take place in rpoH mutants, and probably specific to the metA gene, as another gene of the methionine regulon (metE) was not activated. These results suggest a metabolic link between the two systems that control the response of E. coli to elevated temperatures: the metA gene, which codes for the enzyme responsible for regulating cell growth as a function of temperature elevation (HTS), is transcriptionally activated by the heat shock response.

Compilation of E. coli mRNA promoter sequences

An updated compilation of 300 E. coli mRNA promoter sequences is presented. For each sequence the most recent relevant paper was checked, to verify the location of the transcriptional start position as identified experimentally. We comment on the reliability of the sequence databanks and analyze the conservation of known promoter features in the current compilation. This database is available by E-mail.

Cloning and characterization of the CYS3 (CYI1) gene of Saccharomyces cerevisiae

A DNA fragment containing the Saccharomyces cerevisiae CYS3 (CYI1) gene was cloned. The clone had a single open reading frame of 1,182 bp (394 amino acid residues). By comparison of the deduced amino acid sequence with the N-terminal amino acid sequence of cystathionine gamma-lyase, CYS3 (CYI1) was concluded to be the structural gene for this enzyme. In addition, the deduced sequence showed homology with the following enzymes: rat cystathionine gamma-lyase (41%), Escherichia coli cystathionine gamma-synthase (36%), and cystathionine beta-lyase (25%). The N-terminal half of it was homologous (39%) with the N-terminal half of S. cerevisiae O-acetylserine and O-acetylhomoserine sulfhydrylase. The cloned CYS3 (CYI1) gene marginally complemented the E. coli metB mutation (cystathionine gamma-synthase deficiency) and conferred cystathionine gamma-synthase activity as well as cystathionine gamma-lyase activity to E. coli; cystathionine gamma-synthase activity was detected when O-succinylhomoserine but not O-acetylhomoserine was used as substrate. We therefore conclude that S. cerevisiae cystathionine gamma-lyase and E. coli cystathionine gamma-synthase are homologous in both structure and in vitro function and propose that their different in vivo functions are due to the unavailability of O-succinylhomoserine in S. cerevisiae and the scarceness of cystathionine in E. coli.

Regulation of the Salmonella typhimurium metA gene by the metR protein and homocysteine

The DNA sequence of the Salmonella typhimurium metA control region is presented. S1 nuclease mapping was used to determine the transcription initiation site. By measuring beta-galactosidase levels in Escherichia coli strains lysogenized with lambda phage carrying a metA-lacZ gene fusion, the MetR protein was shown to activate the metA gene. Homocysteine, an intermediate in methionine biosynthesis, plays a negative role in the MetR-mediated activation mechanism. Gel mobility shift assays and DNase I protection experiments showed that the MetR protein binds to a DNA fragment carrying the metA control region and protects a 26-bp region beginning 9 bp upstream of the -35 promoter sequence.

Control site location and transcriptional regulation in Escherichia coli

The regulatory regions for 119 Escherichia coli promoters have been analyzed, and the locations of the regulatory sites have been cataloged. The following observations emerge. (i) More than 95% of promoters are coregulated with at least one other promoter. (ii) Virtually all sigma 70 promoters contain at least one regulatory site in a proximal position, touching at least position -65 with respect to the start point of transcription. There are not yet clear examples of upstream regulation in the absence of a proximal site. (iii) Operators within regulons appear in very variable proximal positions. By contrast, the proximal activation sites of regulons are much more fixed. (iv) There is a forbidden zone for activation elements downstream from approximately position -20 with respect to the start of transcription. By contrast, operators can occur throughout the proximal region. When activation elements appear in the forbidden zone, they repress. These latter examples usually involve autoregulation. (v) Approximately 40% of repressible promoters contain operator duplications. These occur either in certain regulons where duplication appears to be a requirement for repressor action or in promoters subject to complex regulation. (vi) Remote operator duplications occur in approximately 10% of repressible promoters. They generally appear when a multiple promoter region is coregulated by cyclic AMP receptor protein. (vii) Sigma 54 promoters do not require proximal or precisely positioned activator elements and are not generally subject to negative regulation. Rationales are presented for all of the above observations.

Overproduction and characterization of the iclR gene product of Escherichia coli K-12 and comparison with that of Salmonella typhimurium LT2

The iclR gene of Escherichia coli K-12, which encodes a regulatory protein (repressor) for the aceBAK operon, is located between that operon and metH in the 91-min region of the chromosome. The iclR gene was cloned and expressed in a coupled T7 RNA polymerase/promoter system and the gene product was identified by specific binding to a fragment containing the aceBAK operator region. The iclR gene product is a polypeptide of 274 amino acids (aa) with a calculated Mr of 29,741. Comparison of the deduced IclR aa sequence to that of Salmonella typhimurium revealed that the two IclR repressors exhibit 89% identity. A possible helix-turn-helix motif characteristic of DNA-binding proteins was found within the IclR sequence. A search in protein data banks revealed that IclR has a score of similarity of 43.7% with GylR, a transcriptional regulator of the glycerol operon of Streptomyces coelicolor.

The Escherichia coli K-12 metJ193 allele contains a point mutation which alters the hydrophobic pocket responsible for in vitro binding of S-adenosylmethionine: effects on cell growth and induction of met regulon expression

The metJ193 allele encodes one of two identified temperature-sensitive Escherichia coli K-12 met repressors. The nucleotide sequence of the metJ193 allele was determined. The point mutation was a T to A transversion at base 170 of the metJ193 open reading frame and resulted in the substitution of leucine by glutamine at the 56th amino acid residue of the MetJ193 protein. The mutational lesion altered the hydrophobic pocket responsible for in vitro binding of the corepressor S-adenosylmethionine by wild-type MetJ. MetJ193 protein formed at the permissive temperature (28 degrees C) allowed slow derepression of met regulon expression when cultures were shifted to the nonpermissive temperature (34 degrees C). When 28 degrees C cultures of strains bearing two metJ193 alleles were transferred from methionine-containing medium to minimal medium, derepression of met regulon expression did not occur quickly enough to avoid a lag in growth due to the methionine deprivation. The inability of the MetJ193 protein to easily accomplish transition between apo- and active-repressor conformations was also demonstrated by using a maxicell system to study expression of a plasmid-borne copy of the E. coli metF transcription unit. These results confirm the importance of the leucine 56 residue for the structure and function in vivo of the wild-type MetJ protein.

The Escherichia coli regulatory protein MetJ binds to a tandemly repeated 8 bp palindrome

Site-directed oligonucleotide mutagenesis has been used to isolate thirty four new mutants in the regulatory region of the Escherichia coli K12 gene, metF. The mutants include single base pair (bp) substitutions and insertions, double bp substitutions and one 7bp deletion. The effects of these and another five previously described mutants on the transcriptional regulation of metF have been analysed by using a metF'-lac'Z fusion in a low copy-number plasmid. These data, and those obtained from DNAse protection studies using pure MetJ with wild-type and mutant metF operator DNA, show that the metF operator is comprised of five tandem 8 bp repeat units that overlap the -10 region of the metF promoter. In the presence of the co-repressor S-adenosylmethionine, the DNAse protection studies yielded dissociation constants of 150 nM and 300 nM for the interaction of MetJ with repeat units 1 to 4 and repeat unit 5, respectively. In the absence of co-repressor, the dissociation constants obtained for these interactions are four to five times greater. It is proposed that regulation at the metF operator requires four molecules of MetJ dimer to bind to the five 8 bp repeat units to form a tandem, overlapping array. Interactions between MetJ molecules make an essential contribution to the stability of this protein-DNA complex.

Genetic characterization of the gene hupA encoding the HU-2 protein of Escherichia coli

The gene hupA encoding the HU-2 (HU-alpha) protein of Escherichia coli was mapped between rpoBC at 90 min and metA at 90.5 min on the K-12 genome by plasmid integration and P1-mediated transduction studies. Genetic studies using plasmid rescue techniques and physical mapping demonstrated that the gene order is rpoBC-hupA-rrnE-metA. The rpoBC is located approximately 10 kb proximal to the hupA gene, and rrnE about 8 kb distal to it. Thus, the direction of hupA gene transcription is clockwise on the E. coli map [Bachmann B.J., Microbiol. Rev. 47 (1983) 180-230].

Methionine biosynthesis in Enterobacteriaceae: biochemical, regulatory, and evolutionary aspects

The genes coding for the enzymes involved in methionine biosynthesis and regulation are scattered on the Escherichia coli chromosome. All of them have been cloned and most have been sequenced. From the information gathered, one can establish the existence (upstream of the structural genes coding for the biosynthetic genes and the regulatory gene) of "methionine boxes" consisting of two or more repeats of an octanucleotide sequence pattern. The comparison of these sequences allows the extraction of a consensus operator sequence. Mutations in these sequences lead to the constitutivity of the vicinal structural gene. The operator sequence is the target of a DNA-binding protein--the methionine aporepressor--which has been obtained in the pure state, for which S-adenosylmethionine acts as the corepressor. Mutations in the corresponding gene lead to the constitutive expression of all the methionine structural genes. The physicochemical properties of the methionine aporepressor are being investigated.

Analysis of E. coli promoter sequences

We have compiled and analyzed 263 promoters with known transcriptional start points for E. coli genes. Promoter elements (-35 hexamer, -10 hexamer, and spacing between these regions) were aligned by a program which selects the arrangement consistent with the start point and statistically most homologous to a reference list of promoters. The initial reference list was that of Hawley and McClure (Nucl. Acids Res. 11, 2237-2255, 1983). Alignment of the complete list was used for reference until successive analyses did not alter the structure of the list. In the final compilation, all bases in the -35 (TTGACA) and -10 (TATAAT) hexamers were highly conserved, 92% of promoters had inter-region spacing of 17 +/- 1 bp, and 75% of the uniquely defined start points initiated 7 +/- 1 bases downstream of the -10 region. The consensus sequence of promoters with inter-region spacing of 16, 17 or 18 bp did not differ. This compilation and analysis should be useful for studies of promoter structure and function and for programs which identify potential promoter sequences.

Operator-constitutive mutations of the Escherichia coli metF gene

The Escherichia coli metF gene codes for 5,10-methylene-tetrahydrofolate reductase, the enzyme that leads to the formation of N-methyltetrahydrofolate, supplying the methyl group of methionine. Transcription of metF, as well as most of the methionine genes, is repressed by the metJ gene product complexed with S-adenosylmethionine. A metF'-'lacZ gene fusion was used to isolate mutants that have altered expression from the metF promoter. The nucleotide sequences of the metF regulatory region from five such mutants were determined. The mutations were located in the region previously defined as the potential target of the methionine repressor by its similarity to other binding sites. The mutationally defined metF operator thus consists of a 40-base-pair-long region, with five 8-base-pair imperfect palindromes spanning the metF transcription start. The altered operators do not recognize the purified repressor in an in vitro transcription-translation system, although the repressor binds efficiently to the metF wild-type operator.

Mutations affecting the regulation of the metB gene of Salmonella typhimurium LT2

We isolated and characterized cis-acting mutations that affect the regulation of the metB gene of Salmonella typhimurium LT2. The mutations were isolated in an Escherichia coli lac deletion strain lysogenized with lambda bacteriophage carrying a metB-lacZ gene fusion (lambda JBlac) in which beta-galactosidase production is dependent upon metB gene expression. The mutant lysogens show elevated, poorly regulated beta-galactosidase production. The altered regulation is a result of disruption of the methionine control system mediated by the metJ repressor. The mutations are located in a region of dyad symmetry centered near the -35 sequence of the metB promoter. We propose that these mutations alter the repressor binding site and define the metB operator sequence. In addition, we discuss a highly conserved, nonsymmetric DNA sequence of unknown function which occurs in the control regions of the metA, metC, metE, metF, metG, and metJB genes of both S. typhimurium and E. coli.

Control of metF gene expression in maxicell preparations of Escherichia coli K-12: reversible action of the metJ protein and effect of vitamin B12

Expression of methionine regulon elements was controlled by the metJ protein gpMetJ. A maxicell system with cloned copies of the metF transcription unit allowed reversible action of gpMetJ. Expression of the metF transcription unit in maxicells was reduced by exogenous vitamin B12 at concentrations of 0.5 nM or greater.

Regulation of in vivo transcription of the Escherichia coli K-12 metJBLF gene cluster

We subcloned DNA of the intercistronic region between the divergently transcribed metJ and metB genes of Escherichia coli into the transcription-fusion vector pK01 and localized the metJ promoters by deletion analysis. The plasmid-borne promoters of both genes were repressed by chromosomal metJ. In addition, S1 nuclease mapping of chromosomally derived mRNA from a derepressed strain revealed the start sites of transcription for metBL, metF, and metJ. The metBL and metF genes each had a single transcript which was repressed by metJ, while the metJ gene had three transcripts, of which the first was strongly repressed by metJ, the second was less strongly repressed, and the third was not repressed.

Evolution in biosynthetic pathways: two enzymes catalyzing consecutive steps in methionine biosynthesis originate from a common ancestor and possess a similar regulatory region

The metC gene of Escherichia coli K-12 was cloned and the nucleotide sequence of the metC gene and its flanking regions was determined. The translation initiation codon was identified by sequencing the NH2-terminal part of beta-cystathionase, the MetC gene product. The metC gene (1185 nucleotides) encodes a protein having 395 amino acid residues. The 5' noncoding region was found to contain a "Met box" homologous to sequences suggestive of operator structures upstream from other methionine genes that are controlled by the product of the pleiotropic regulatory metJ gene. The deduced amino acid sequence of beta-cystathionase showed extensive homology with that of the MetB protein (cystathionine gamma-synthase) that catalyzes the preceding step in methionine biosynthesis. The homology strongly suggests that the structural genes for the MetB and MetC proteins evolved from a common ancestral gene.

Promoter selectivity of E. coli RNA polymerase: analysis of the promoter system of convergently-transcribed dnaQ-rnh genes

Promoter properties were analyzed for the convergently-overlapped E. coli genes coding for the DNA polymerase III epsilon subunit (dnaQ) and the ribonuclease H (rnh). The rates of open complex formation for a single promoter of the rnh gene and two tandem promoters of the dnaQ gene were constant whether they are located on a single DNA fragment or separated into individual fragments. The relative expression levels of these three promoters, as measured using an in vitro mixed transcription system, varied differentially depending on the concentration of RNA polymerase. At low enzyme concentrations, the downstream promoter (P2) of the dnaQ gene was utilized preferentially, but the upstream promoter (P1) was utilized as well when the enzyme concentration was increased. This indicates different physiological roles between the two dnaQ promoters. The level of rnh transcription was as low as that of dnaQ-1 RNA synthesis but the rnh promoter was utilized as well as the dnaQ P2 promoter when it was separated from the dnaQ promoters. This implies a promoter interference between the convergently transcribed genes.

Isolation and characterization of the product of the methionine-regulatory gene metJ of Escherichia coli K-12

We have modified a previously isolated metJ plasmid by removing a segment of DNA including the rop gene. Bacterial strains carrying this plasmid produce elevated levels of the metJ gene product, presumably because of the high number of gene copies in the cell. We have isolated the metJ gene product in nearly homogeneous form from such a strain. The subunit size and the amino acid composition are the same as those predicted from the DNA sequence of the metJ gene. Sedimentation equilibrium measurements show that the native metJ gene product is a dimer. The purified dimer protects a short segment of DNA in the regulatory region of the metB and metJ genes from hydrolysis by DNase I.