An Anhydro- N -Acetylmuramyl- l -Alanine Amidase with Broad Specificity Tethered to the Outer Membrane of Escherichia coli

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Abstract
From its amino acid sequence homology with AmpD, we recognized YbjR, now renamed AmiD, as a possible second 1,6-anhydro- N -acetylmuramic acid (anhMurNAc)- l -alanine amidase in Escherichia coli . We have now confirmed that AmiD is an anhMurNAc- l -Ala amidase and demonstrated that AmpD and AmiD are the only enzymes present in E. coli that are able to cleave the anhMurNAc- l -Ala bond. The activity was present only in the outer membrane fraction obtained from an ampD mutant. In contrast to AmpD, which is specific for the anhMurNAc- l -alanine bond, AmiD also cleaved the bond between MurNAc and l -alanine in both muropeptides and murein sacculi. Unlike the periplasmic murein amidases, AmiD did not participate in cell separation. ampG mutants, which are unable to import GlcNAc-anhMurNAc-peptides into the cytoplasm, released mainly peptides into the medium due to AmiD activity, whereas an ampG amiD double mutant released a large amount of intact GlcNAc-anhMurNAc-peptides into the medium.