Abstract
Escherichia coli has five genes encoding L,D-transpeptidases (Ldt) with varied functions. Three of these enzymes (YbiS, ErfK, YcfS) have been shown to cross-link Braun's lipoprotein to the peptidoglycan (PG), while the other two (YnhG, YcbB) form direct meso-diaminopimelate (DAP-DAP, or 3-3) cross-links within the PG. In addition, Ldt enzymes can also incorporate non-canonical D-amino acids, such as D-methionine, into the PG. To further investigate the role of these enzymes and, in particular, 3-3 linkages in cell envelope physiology we constructed and phenotypically characterized a variety of multiple Ldt deletion mutants of E. coli. We report that a triple deletion mutant lacking ybiS, erfK and ycfS is hypersusceptible to the metal-chelating agent EDTA, leaks periplasmic proteins and is resistant to the toxic effect of D-methionine. A double ynhG ycbB mutant had no discernible phenotype; however, examination of the phenotypes of various Ldt mutants bearing an additional DAP auxotrophic mutation (dapA : : Cm) showed that a quintuple mutant strain lacking all Ldt genes was severely impaired for growth on media with limited DAP. These data demonstrate that loss of the E. coli Ldt enzymes involved with coupling the PG to Braun's lipoprotein resulted in the loss of outer membrane stability while loss of the Ldt enzymes involved with DAP-DAP linkages had no observable effect on the cell envelope. Loss of all Ldt enzymes proved detrimental to growth when cells were starved for DAP, indicating a combined role for both 3-3 and Braun's lipoprotein cross-links in cell viability only under a specific PG stress.