Identification of MltG as a potential terminase for peptidoglycan polymerization in bacteria

Abstract

SUMMARY Bacterial cells are fortified against osmotic lysis by a cell wall made of peptidoglycan (PG). Synthases called penicillin-binding proteins (PBPs), the targets of penicillin and related antibiotics, polymerize the glycan strands of PG and crosslink them into the cell wall meshwork via attached peptides. The average length of glycan chains inserted into the matrix by the PBPs is thought to play an important role in bacterial morphogenesis, but polymerization termination factors controlling this process have yet to be discovered. Here, we report the identification of Escherichia coli MltG (YceG) as a potential terminase for glycan polymerization that is broadly conserved in bacteria. A clone containing mltG was initially isolated in a screen for multicopy plasmids generating a lethal phenotype in cells defective for the PG synthase PBP1b. Biochemical studies revealed that MltG is an inner membrane enzyme with endolytic transglycosylase activity capable of cleaving at internal positions within a glycan polymer. Radiolabeling experiments further demonstrated MltG-dependent nascent PG processing in vivo, and bacterial two-hybrid analysis identified an MltG-PBP1b interaction. Mutants lacking MltG were also shown to have longer glycans in their PG relative to wild-type cells. Our combined results are thus consistent with a model in which MltG associates with PG synthetic complexes to cleave nascent polymers and terminate their elongation.

SagB Glucosaminidase Is a Determinant of Staphylococcus aureus Glycan Chain Length, Antibiotic Susceptibility, and Protein Secretion

“…Studying purified PGTs from E. coli (PBP1A), S. aureus (PBP2), and Enterococcus faecalis, Wang et al observed that each PGT generated polymers with characteristic lengths, independently of enzyme/substrate ratios, and proposed that PGTs may rely on an intrinsic mechanism for controlling product length (64). Mutations in the structural gene for mltG, encoding a lytic transglycosylase of the YceG family (Pfam02618) in E. coli, increase the length of glycan strands in the peptidoglycan (52). As MltG is thought to associate with PBP1B, an E. coli PGT, Yunck and colleagues proposed an alternative model, namely, that lytic transglycosylases of the YceG family may terminate glycan chain polymerization during peptidoglycan synthesis (52).…”

“…Mutations in the structural gene for mltG, encoding a lytic transglycosylase of the YceG family (Pfam02618) in E. coli, increase the length of glycan strands in the peptidoglycan (52). As MltG is thought to associate with PBP1B, an E. coli PGT, Yunck and colleagues proposed an alternative model, namely, that lytic transglycosylases of the YceG family may terminate glycan chain polymerization during peptidoglycan synthesis (52). The genome of S. aureus does not encode lytic YceGtype transglycosylases, suggesting that S. aureus must have evolved another mechanism to generate the characteristically short glycan strands of its peptidoglycan layer (17).…”

“…Changes in the structure of the cell envelope have been described to affect bacterial susceptibility to antibiotics (50). For example, mutations in E. coli mltG, which encodes a lytic transglycosylase, gives rise to peptidoglycan with elongated glycan strands and causes increased susceptibility to ␤-lactam antibiotics (51,52). S. aureus glucosaminidase variants as well as the atl sagAB scaH mutant strain were tested for susceptibility to flavomycin, a glycosyltransferase inhibitor (53), and oxacillin, a ␤-lactamaseresistant ␤-lactam antibiotic that, unlike the related antibiotic methicillin, displays peroral drug activity (54).…”

Cell-wall recycling and synthesis in Escherichia coli and Pseudomonas aeruginosa – their role in the development of resistance

“…Investigation of the E. coli open reading frames led to the identification of eight genes encoding LTs; mltA, mltB, mltC, mltD, mltE (emtA), mltF, mltG and slt70 [81,[85][86][87][88][89][90].…”

Turnover of Bacterial Cell Wall by SltB3, a Multidomain Lytic Transglycosylase of Pseudomonas aeruginosa

“…These enzymes perform important biological functions, which are the subjects of intense study. The importance of their functions is underscored by their multiplicity-Escherichia coli has eight [3][4][5] and Pseudomonas aeruginosa has eleven 6 -and by redundancies of the reactions that they perform. For example, six of the enzymes of E. coli could be ablated without any apparent consequence, but the organism cannot survive the loss of seven.…”