Dihydroorotase from Escherichia coli. Cloning the pyrC gene and production of tryptic peptide maps

Mass spectrometric determination of the cleavage sites in Escherichia coli dihydroorotase induced by a cysteine-specific reagent

Escherichia coli dihydroorotase contains six cysteines/subunit, which are potential ligands of structural and catalytic zinc metals at protein sites of the enzyme. Specific thiol reagents modify, in nondenaturing conditions only, two of these cysteines; these two residues are thought to be ligands of structural zinc. We report here on the localization of these two cysteines on the polypeptide chain through their cyanylation by 2-nitro-5-thiocyanobenzoic acid (NTCB) and the analysis by mass spectrometry of the protein adducts. This is the first study of E. coli dihydroorotase by mass spectrometry, allowing the accurate determination of the subunit molecular weight (38,695). Treatment of dihydroorotase by NTCB induced a cleavage N-terminal to the cyanylated cysteines. The resulting fragments visualized on electrophoresis gel have been N-terminal sequenced, and their masses were determined by electrospray-ionizing mass spectrometry. This allowed the identification of cysteines 221 and 265 as the two residues cyanylated by the reagent NTCB. Results from gel filtration of dihydroorotase cyanylated on the two cysteines indicate that these residues are involved in subunit interactions leading to the active dimer. Consistent with literature data, we assume that cysteine 221 and cysteine 265, along with the neighboring cysteines 263 and 268 arranged in cluster, are potential ligands of structural zinc of E. coli dihydroorotase.

The mdoA locus of Escherichia coli consists of an operon under osmotic control

In Escherichia coli, the 5 kb mdoA locus is involved in the osmotically controlled biosynthesis of periplasmic membrane-derived oligosaccharides (MDOs). The structure of this locus was analysed by in vitro cassette insertion, transposon mutagenesis, and gene-fusion analysis. A 'neo' cassette, derived from the neomycin phosphotransferase II region of transposon Tn5, was inserted into mdoA, borne by a multicopy plasmid. This plasmid was shown to complement two previously described mdoA mutations, depending on the orientation of the exogenous gene. Thus, the gene altered by these mutations could be expressed under the control of the exogenous promoter. Moreover, the 'neo' cassette inactivated another, uncharacterized, mdo gene, because when this insertion was transferred into the chromosome MDO synthesis was abolished. The existence of a second gene was confirmed by complementation analysis with a collection of Tn1000 insertions into mdoA. Two groups were defined, and the two genes are organized into an operon (mdoGH). This conclusion was reached because Tn1000 insertions in the first gene displayed a polar effect on the expression of the second gene. An active gene fusion was obtained on a multicopy plasmid between the beginning of mdoH and lacZ. The hybrid beta-galactosidase activity followed the same osmotically controlled response as that described for of MDO synthesis. This regulation was unaffected by the presence, or absence, of MDOs in the periplasm. Finally, the amount of mdoA-specific mRNAs, determined by dot blot hybridization, decreased when the osmolarity of the growth medium increased.

Molecular cloning and expression of a locus (mdoA) implicated in the biosynthesis of membrane-derived oligosaccharides in Escherichia coli

Mutants of Escherichia coli defective in the mdoA locus are blocked at an early stage in the biosynthesis of membrane-derived oligosaccharides. The mdoA locus has now been cloned into multicopy plasmids. A 5 kb DNA fragment is necessary to complement mdoA mutations. Cells harbouring the mdoA+ plasmid produced three to four times more MDO than wild-type cells. MDO overproduction did not affect the degree of MDO substitution with sn-1-phosphoglycerol residues. The biosynthesis of MDO remained under osmotic control in overproducing strains.