Data from: Factors essential for L,D-transpeptidase-mediated peptidoglycan cross-linking and β-lactam resistance in Escherichia coli

The target of β-lactam antibiotics is the D,D-transpeptidase activity of
penicillin-binding proteins (PBPs) for synthesis of 4→3 cross-links in the
peptidoglycan of bacterial cell walls. Unusual 3→3 cross-links formed by
L,D-transpeptidases were first detected in Escherichia coli more than four
decades ago, however no phenotype has previously been associated with
their synthesis. Here we show that production of the L,D-transpeptidase
YcbB in combination with elevated synthesis of the (p)ppGpp alarmone by
RelA lead to full bypass of the D,D-transpeptidase activity of PBPs and to
broad-spectrum β-lactam resistance. Production of YcbB was therefore
sufficient to switch the role of (p)ppGpp from antibiotic tolerance to
high-level β-lactam resistance. This observation identifies a new mode of
peptidoglycan polymerization in E. coli that relies on an unexpectedly
small number of enzyme activities comprising the glycosyltransferase
activity of class A PBP1b and the D,D-carboxypeptidase activity of DacA in
addition to the L,D-transpeptidase activity of YcbB.

β-内酰胺类抗生素（β-lactam antibiotics）的作用靶点为青霉素结合蛋白（penicillin-binding proteins, PBPs）的D,D-转肽酶（D,D-transpeptidase）活性，该活性负责介导细菌细胞壁肽聚糖内4→3交联键的合成。由L,D-转肽酶（L,D-transpeptidase）催化形成的罕见3→3交联键，于四十余年前首次在大肠杆菌（Escherichia coli）中被检测到，但此前尚无研究将此类交联键的合成与特定表型相关联。本研究发现，当表达L,D-转肽酶YcbB，同时RelA介导的(p)ppGpp警报素（(p)ppGpp alarmone）合成水平升高时，可完全绕过PBPs的D,D-转肽酶活性，进而使菌株获得广谱β-内酰胺类抗生素耐药性。因此，仅表达YcbB即可将(p)ppGpp的功能从介导抗生素耐受转变为赋予菌株高水平的β-内酰胺类抗生素耐药性。本研究结果揭示了大肠杆菌内一种全新的肽聚糖聚合模式，该模式仅依赖于数量少得出乎意料的酶活性：除YcbB的L,D-转肽酶活性外，还包括A类青霉素结合蛋白1b（class A PBP1b）的糖基转移酶（glycosyltransferase）活性，以及DacA的D,D-羧肽酶（D,D-carboxypeptidase）活性。