Cloning and expression of the metE gene in Escherichia coli

The effects of methionine acquisition and synthesis on Streptococcus pneumoniae growth and virulence

Bacterial pathogens need to acquire nutrients from the host, but for many nutrients their importance during infection remain poorly understood. We have investigated the importance of methionine acquisition and synthesis for Streptococcus pneumoniae growth and virulence using strains with gene deletions affecting a putative methionine ABC transporter lipoprotein (Sp_0149, metQ) and/or methionine biosynthesis enzymes (Sp_0585 - Sp_0586, metE and metF). Immunoblot analysis confirmed MetQ was a lipoprotein and present in all S. pneumoniae strains investigated. However, vaccination with MetQ did not prevent fatal S. pneumoniae infection in mice despite stimulating a strong specific IgG response. Tryptophan fluorescence spectroscopy and isothermal titration calorimetry demonstrated that MetQ has both a high affinity and specificity for L-methionine with a K_D of ∼25 nM, and a ΔmetQ strain had reduced uptake of C₁₄-methionine. Growth of the ΔmetQ/ΔmetEF strain was greatly impaired in chemically defined medium containing low concentrations of methionine and in blood but was partially restored by addition of high concentrations of exogenous methionine. Mixed infection models showed no attenuation of the ΔmetQ, ΔmetEF and ΔmetQ/ΔmetEF strains in their ability to colonise the mouse nasopharnyx. In a mouse model of systemic infection although significant infection was established in all mice, there were reduced spleen bacterial CFU after infection with the ΔmetQ/ΔmetEF strain compared to the wild-type strain. These data demonstrate that Sp_0149 encodes a high affinity methionine ABC transporter lipoprotein and that Sp_0585 – Sp_0586 are likely to be required for methionine synthesis. Although Sp_0149 and Sp_0585-Sp_0586 make a contribution towards full virulence, neither was essential for S. pneumoniae survival during infection.

Sinorhizobium meliloti requires a cobalamin-dependent ribonucleotide reductase for symbiosis with its plant host

Vitamin B(12) (cobalamin) is a critical cofactor for animals and protists, yet its biosynthesis is limited to prokaryotes. We previously showed that the symbiotic nitrogen-fixing alphaproteobacterium Sinorhizobium meliloti requires cobalamin to establish a symbiotic relationship with its plant host, Medicago sativa (alfalfa). Here, the specific requirement for cobalamin in the S. meliloti-alfalfa symbiosis was investigated. Of the three known cobalamin-dependent enzymes in S. meliloti, the methylmalonyl CoA mutase (BhbA) does not affect symbiosis, whereas disruption of the metH gene encoding the cobalamin-dependent methionine synthase causes a significant defect in symbiosis. Expression of the cobalamin-independent methionine synthase MetE alleviates this symbiotic defect, indicating that the requirement for methionine synthesis does not reflect a need for the cobalamin-dependent enzyme. To investigate the function of the cobalamin-dependent ribonucleotide reductase (RNR) encoded by nrdJ, S. meliloti was engineered to express an Escherichia coli cobalamin-independent (class Ia) RNR instead of nrdJ. This strain is severely defective in symbiosis. Electron micrographs show that these cells can penetrate alfalfa nodules but are unable to differentiate into nitrogen-fixing bacteroids and, instead, are lysed in the plant cytoplasm. Flow cytometry analysis indicates that these bacteria are largely unable to undergo endoreduplication. These phenotypes may be due either to the inactivation of the class Ia RNR by reactive oxygen species, inadequate oxygen availability in the nodule, or both. These results show that the critical role of the cobalamin-dependent RNR for survival of S. meliloti in its plant host can account for the considerable resources that S. meliloti dedicates to cobalamin biosynthesis.

Vitamin-B12-independent methionine synthase from a higher plant (Catharanthus roseus). Molecular characterization, regulation, heterologous expression, and enzyme properties

Methionine synthases catalyze the formation of methionine by the transfer of a methyl group from 5-methyltetrahydrofolate to homocysteine. This reaction is the last step in L-methionine biosynthesis, and it also serves to regenerate the methyl group of S-adenosylmethionine, a cofactor required for biological methylation reactions. We describe the cloning, expression and characterization of a methionine synthase from the higher plant Catharanthus roseus. cDNAs were identified that encoded a protein of 85 kDa sharing 50% identify with the cobalamin-independent methionine synthase from Escherichia coli (MetE) and 41% identity with a partial sequence of a yeast homolog of MetE. The C. roseus protein was expressed at high levels in E. coli. The enzyme accepts the triglutamate form of methyltetrahydrofolate as a methyl donor but not the monoglutamate form, and it does not require S-adenosylmethionine or cobalamin for activity. The properties indicate that the enzyme is a cobalamin-independent methionine synthase (EC 2.1.1.14). In contrast to the E. coli MetE, the plant protein does not require phosphate or magnesium ions for activity. Immunoblots of plants extracts showed that the protein was localized in the cytosol, and was present in a variety of plant species. A nutritional downshift of the C. roseus cell culture revealed a strong, transient transcriptional activation, but no significant increment in the total level of the protein. The availability of the protein and the cDNA now provide tools to investigate the complexities of methionine biosynthesis in plants.

Regulation of methionine synthesis in Escherichia coli

The biosynthesis of methionine in Escherichia coli is under complex regulation. The repression of the biosynthetic pathway by methionine is mediated by a repressor protein (MetJ protein) and S-adenosyl-methionine which functions as a corepressor for the MetJ protein. Recently, a new regulatory locus, metR, has been identified. The MetR protein is required for both metE and metH gene expression, and functions as a transactivator of transcription of these genes. MetR is a unique prokaryotic transcription activator in that it possesses a leucine zipper motif, first described for eukaryotic DNA-binding proteins. The transcriptional activity of MetR is modulated by homocysteine, the metabolic precursor of methionine. Finally, it is known that vitamin B12 can repress expression of the metE gene. This effect is mediated by the MetH holoenzyme, which contains a cobamide prosthetic group.

Nucleotide sequence of the metH gene of Escherichia coli K-12 and comparison with that of Salmonella typhimurium LT2

The Escherichia coli K-12 metH gene, encoding the vitamin B12-dependent homocysteine transmethylase, is located between iclR and lysC in the 91-min region of the chromosome. The metH gene has been sequenced and reveals an open reading frame of 3600 bp encoding a polypeptide of 1200 amino acids (aa) with a calculated Mr of 132 628. The first 414 aa of the deduced polypeptide sequence are 92% identical to the 414 aa deduced from the partially sequenced Salmonella typhimurium LT2 metH gene. In-frame fusions of metH to lacZ were used to confirm the reading frame of the metH gene and to study its regulation. metH was repressed tenfold, presumably indirectly, by L-methionine and the metJ gene product, while vitamin B12 did not induce de novo synthesis of MetH.

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.

Methionine synthesis in Escherichia coli: effect of the MetR protein on metE and metH expression

Studies by Urbanowski et al. [Urbanowski, M. L., Stauffer, L. T., Plamann, L. S. & Stauffer, G. V. (1987) J. Bacteriol. 169, 1391-1397] have identified a regulatory locus, called metR, required for the expression of the metE and metH genes. We recently purified the MetR protein from Escherichia coli and showed that it could stimulate the in vitro expression of the metE gene and autoregulate its own synthesis. In the present study, the purified MetR protein has been shown to stimulate the in vitro expression of the metH gene. Also, the in vitro synthesized MetE, MetH, and MetR proteins were shown to be biologically active. The transcription start sites for the metE and metR genes have been determined, and DNA footprinting experiments have identified regions in the metE-metR intergenic sequence that are protected by either the MetR or MetJ proteins.

Regulation of methionine synthesis in Escherichia coli: effect of the MetR protein on the expression of the metE and metR genes

A plasmid (pRSE562) containing the metE and metR genes of Escherichia coli was used to study the expression of these genes and the role of the MetR protein in regulating metE expression. DNA sequence analysis of the 236-base-pair region separating these genes showed the presence of seven putative met boxes. When this plasmid was used to transform either wild-type E. coli, metE mutant, or metR mutant, MetE enzyme activity increased 5- to 7-fold over wild-type levels. The metR gene was subcloned from pRSE562, and this plasmid, pMRIII, relieved the methionine auxotrophy of a metR mutant after transformation. The metR gene was also cloned into a vector containing the lambda PL promoter, and the MetR protein was overexpressed and purified to near homogeneity. This protein, when added to an in vitro DNA-dependent protein synthesis system in which the MetE and/or MetR proteins were synthesized, caused a large increase in the expression of the metE gene but a decrease in the expression of the metR gene. The in vitro expression of both genes was inhibited by the MetJ protein and S-adenosylmethionine in the presence or absence of MetR protein. These results provide evidence that the product of the metR gene is a trans-activator of the expression of the metE gene and that the expression of the metR gene is under autogenous regulation and is repressed by the MetJ protein.

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.

Cloning and characterization of the genes for the two homocysteine transmethylases of Escherichia coli

We have cloned the genes for the two homocysteine transmethylases of Escherichia coli K12. The vitamin B12-independent enzyme is encoded by the metE gene while the metH gene codes for the vitamin B12-requiring enzyme. Overexpression of the gene products and Tn1000 mutagenesis have enabled the metE and metH gene products to be identified as 99 kDa and 130 kDa polypeptides, respectively. The truncated polypeptides generated by Tn1000 insertion were used to determine the direction of transcription of the metE and metH genes. Negative complementation suggests that the MetH enzyme exists as an oligomer. Investigation of the expression of the chromosomal- and plasmid-encoded gene products confirms that metE is subject to negative control by vitamin B12 and methionine, and that metH is under positive control by the cofactor and negative control by methionine. For vitamin B12 and methionine to act as regulatory effectors in metE control, functional metH and metJ genes are required, respectively. The use of stable Tn1000-generated fragments of the metE product as electrophoretic markers for the plasmid-encoded metE gene product demonstrated that the two regulatory proteins involved in negative control of metE are present in excess. Under conditions whereby both forms of negative metE control are non-functional, the metE gene product represented about 90% of the total protein, and cell growth was severely impaired.

Molecular cloning, characterization, and chromosomal localization of dapF, the Escherichia coli gene for diaminopimelate epimerase

The Escherichia coli dapF gene was isolated from a cosmid library as a result of screening for clones overproducing diaminopimelate epimerase. Insertional mutagenesis was performed on the cloned dapF gene with a mini-Mu transposon, leading to chloramphenicol resistance. One of these insertions was transferred onto the chromosome by a double-recombination event, allowing us to obtain a dapF mutant. This mutant accumulated large amounts of LL-diaminopimelate, confirming the blockage in the step catalyzed by the dapF product, but did not require meso-diaminopimelate for growth. The dapF gene was localized in the 85-min region of the E. coli chromosome between cya and uvrD.