Tsui Ho-Ching Tiffany, Zheng Jiaqi J, Magallon Ariel N, Ryan John D, Yunck Rachel, Rued Britta E, Bernhardt Thomas G, Winkler Malcolm E
Department of Biology, Indiana University Bloomington, Bloomington, IN, 47405, USA.
Department of Microbiology and Immunology, Harvard Medical School, Boston, MA, 02115, USA.
Mol Microbiol. 2016 Jun;100(6):1039-65. doi: 10.1111/mmi.13366. Epub 2016 Apr 15.
In ellipsoid-shaped ovococcus bacteria, such as the pathogen Streptococcus pneumoniae (pneumococcus), side-wall (peripheral) peptidoglycan (PG) synthesis emanates from midcells and is catalyzed by the essential class B penicillin-binding protein PBP2b transpeptidase (TP). We report that mutations that inactivate the pneumococcal YceG-domain protein, Spd_1346 (renamed MltG), remove the requirement for PBP2b. ΔmltG mutants in unencapsulated strains accumulate inactivation mutations of class A PBP1a, which possesses TP and transglycosylase (TG) activities. The 'synthetic viable' genetic relationship between Δpbp1a and ΔmltG mutations extends to essential ΔmreCD and ΔrodZ mutations that misregulate peripheral PG synthesis. Remarkably, the single MltG(Y488D) change suppresses the requirement for PBP2b, MreCD, RodZ and RodA. Structural modeling and comparisons, catalytic-site changes and an interspecies chimera indicate that pneumococcal MltG is the functional homologue of the recently reported MltG endo-lytic transglycosylase of Escherichia coli. Depletion of pneumococcal MltG or mltG(Y488D) increases sphericity of cells, and MltG localizes with peripheral PG synthesis proteins during division. Finally, growth of Δpbp1a ΔmltG or mltG(Y488D) mutants depends on induction of expression of the WalRK TCS regulon of PG hydrolases. These results fit a model in which MltG releases anchored PG glycan strands synthesized by PBP1a for crosslinking by a PBP2b:RodA complex in peripheral PG synthesis.
在椭圆形的卵球菌属细菌中，如病原菌肺炎链球菌（肺炎球菌），侧壁（外周）肽聚糖（PG）合成从中细胞起始，并由必需的B类青霉素结合蛋白PBP2b转肽酶（TP）催化。我们报告称，使肺炎球菌YceG结构域蛋白Spd_1346（重命名为MltG）失活的突变消除了对PBP2b的需求。非包膜菌株中的ΔmltG突变体积累了具有TP和转糖基酶（TG）活性的A类PBP1a的失活突变。Δpbp1a和ΔmltG突变之间的“合成可行”遗传关系延伸至错误调节外周PG合成的必需ΔmreCD和ΔrodZ突变。值得注意的是，单一的MltG(Y488D)变化抑制了对PBP2b、MreCD、RodZ和RodA的需求。结构建模与比较、催化位点变化以及种间嵌合体表明，肺炎球菌MltG是最近报道的大肠杆菌MltG内切溶菌转糖基酶的功能同源物。肺炎球菌MltG或mltG(Y488D)的缺失增加了细胞的球形度，并且MltG在分裂期间与外周PG合成蛋白共定位。最后，Δpbp1a ΔmltG或mltG(Y488D)突变体的生长取决于PG水解酶的WalRK双组分系统调控子表达的诱导。这些结果符合一个模型，即MltG释放由PBP1a合成的锚定PG聚糖链，以供PBP2b:RodA复合物在外周PG合成中进行交联。
