The MraY translocase catalyzes the first membrane step of bacterial cell wall peptidoglycan synthesis (i.e. the transfer of the phospho-N-acetylmuramoyl-pentapeptide motif onto the undecaprenyl phosphate carrier lipid), a reversible reaction yielding undecaprenylpyrophosphoryl-N-acetylmuramoyl-pentapeptide (lipid intermediate I). This essential integral membrane protein, which is considered as a very promising target for the search of new antibacterial compounds, has thus far been clearly underexploited due to its intrinsic refractory nature to overexpression and purification. We here report conditions for the high level overproduction and for the first time the purification to homogeneity of milligram quantities of MraY protein. The kinetic parameters and effects of pH, salts, cations, and detergents on enzyme activity are described, taking the Bacillus subtilis MraY translocase as a model.

SummaryIn Escherichia coli many enzymes including MurG are directly involved in the synthesis and assembly of peptidoglycan. MurG is an essential glycosyltransferase catalysing the last intracellular step of peptidoglycan synthesis. To elucidate its role during elongation and division events, localization of MurG using immunofluorescence microscopy was performed. MurG exhibited a random distribution in the cell envelope with a relatively higher intensity at the division site. This mid-cell localization was dependent on the presence of a mature divisome. Its localization in the lateral cell wall appeared to require the presence of MreCD. This could be indicative of a potential interaction between MurG and other proteins. Investigating this by immunoprecipitation revealed the association of MurG with MreB and MraY in the same protein complex. In view of this, the loss of rod shape of DmreBCD strain could be ascribed to the loss of MurG membrane localization. Consequently, this could prevent the localized supply of the lipid II precursor to the peptidoglycan synthesizing machinery involved in cell elongation. It is postulated that the involvement of MurG in the peptidoglycan synthesis concurs with two complexes, one implicated in cell elongation and the other in division. A model representing the first complex is proposed.

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Many species of gram-positive bacteria produce branched peptidoglycan precursors resulting from the transfer of various L-amino acids or glycine from amino acyl-tRNA to the -amino group of L-lysine. The UDP-MurNAc-pentapeptide:L-alanine ligase and alanyl-tRNA synthetase genes from Enterococcus faecalis were identified, cloned, and overexpressed in Escherichia coli. The purified enzymes were necessary and sufficient for tRNA-dependent addition of L-alanine to UDP-MurNAc-pentapeptide in vitro. The ligase belonged to the Fem family of proteins, which were initially identified genetically as factors essential for methicillin resistance in Staphylococcus aureus.In gram-positive bacteria, the ε-amino group of L-lysine in the pentapeptide stem of peptidoglycan precursors is often replaced by amino acid chains of various lengths and compositions (19) (Fig. 1). These amino acids form cross bridges between L-Lys 3 and D-Ala 4 following cross-linking of the peptide stems from nascent chains of peptidoglycan by the D,Dtranspeptidases. The ligases for the addition of glycine and L-amino acids to the ε-amino group of L-lysine use aminoacyltRNAs as substrates, whereas D-amino acids are added in a tRNA-independent reaction (15,20).In Staphylococcus aureus, the femA, femB, and fmhB genes were shown to be essential for incorporation of glycine into the side chain of peptidoglycan precursors (18, 21). The femAB locus was initially identified as a factor essential for methicillin resistance (fem) based on random insertional inactivation of chromosomal genes and a screen for reduced expression of resistance mediated by the penicillin binding protein 2A (PBP2A) (2, 5). Inactivation of femA or femB was subsequently reported to prevent incorporation of glycine residues at positions 2 to 5 or positions 4 to 5 of the penta-glycine cross bridge since muropeptides cross-linked by one or three glycine residues were detected in the corresponding mutants (4, 21). Inactivation of fmhB, formerly femX, is lethal, but the construction of a mutant conditionally expressing fmhB under the control of a xylose-inducible promoter showed that the gene was essential for synthesis of branched peptidoglycan precursors (18). These results indicated that the fem gene products were required for incorporation of glycine at positions 1 (FmhB), 2 and 3 (FemA), and 4 and 5 (FemB) of the cross bridge, although the catalytic activity of the proteins was not directly assessed (18). Similarly, inactivation of two fmhB homologues in Streptococcus pneumoniae, designated murM (fibA) and murN (fibB), reduced addition of L-Ala or L-Ser to the ε-amino group of L-Lys and subsequent addition of a second L-Ala residue, respectively (6, 7, 23). Overall, disruption of the murMN operon reduced the proportion of branched peptide stems in the peptidoglycan from 89 to 33% (6). In contrast to what occurs in S. aureus (18), direct cross-linking of L-Lys to D-Ala occurs in S. pneumoniae, and the murMN operon was accordingly reported to be unessential (6). However, production of branch...

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Plasmids for the high-level overproduction of wildtype, and C-and N-terminal His-tagged MurG N-acetylglucosaminyl transferase from Escherichia coli were constructed. In complementation tests the three forms were active in vivo. After IPTG induction, growth, spheroplast formation and lysis, overproduced MurG proteins were mainly present (90%) in the particulate fraction. Readily solubilized by CHAPS, they were purified without any detergent to over 80% purity for both Histagged forms but only up to 20% for the wild-type form. The enzymatic activity of each purified MurG protein was determined and found to be inhibited to the same extent by ramoplanin.z 1999 Federation of European Biochemical Societies.

(1), an analogue of lipid I of peptidoglycan biosynthesis, was synthesized from natural UDP-N-acetylmuramoylpentapeptide in three steps. Compound I was shown to be a substrate for the MurG transferase from Escherichia coli, even in the absence of membranes. When membranes were present, dansylated peptidoglycan was also formed.