Pseudomonas syringae Self-Protection from Tabtoxinine-β-Lactam by Ligase TbIF and Acetylase Ttr

Plant pathogenic Pseudomonas syringae produce the hydroxy-beta-lactam antimetabolite tabtoxinine-beta-lactam (TA) 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 Tb1F which converts tabtoxinine-beta-lactam to the T beta L-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 T beta L is released by peptidase action. The second self-protection mode is elaboration by the acetyltransferase Ttr, which acetylates the alpha-amino group of the proximal inactivator T beta L, but not the tabtoxin dipeptide.