Pseudomonas syringae self-protection from tabtoxinine-β-lactam by ligase TblF and acetylase Ttr

Plant pathogenic Pseudomonas syringae produce the hydroxy-β-lactam antimetabolite tabtoxinine-β-lactam (TβL) as a time-dependent inactivating glutamine analogue of plant glutamine synthetases. The producing pseudomonads use multiple modes of self-protection, two of which are characterized in this study. The first is the dipeptide ligase TblF which converts tabtoxinine-β-lactam to the TβL-Thr dipeptide known as tabtoxin. The dipeptide is not recognized by glutamine synthetase. This represents a Trojan Horse strategy: the dipeptide is secreted, taken up by dipeptide permeases in neighboring cells, and TβL is released by peptidase action. The second self-protection mode is elaboration by the acetyltransferase Ttr, which acetylates the α-amino group of the proximal inactivator TβL, but not the tabtoxin dipeptide.