Crystal structure of halogenase PltA from the pyoluteorin biosynthetic pathway

Pyoluteorin is an antifungal agent composed of a 4,5-dichlorinated pyrrole group linked to a resorcinol moiety. The pyoluteorin biosynthetic gene cluster in Pseudomonas fluorescens Pf-5 encodes the halogenase PltA, which has been previously demonstrated to perform both chlorinations in vitro. PltA selectively accepts as a substrate a pyrrole moiety covalently tethered to a nonribosomal peptide thiolation domain PltL (pyrrolyl-S-PltL) for FAD-dependent di-chlorination, yielding 4,5-dichloropyrrolyl-S-PltL. We report a 2.75 Å-resolution crystal structure of PltA in complex with FAD and chloride. PltA is a dimeric enzyme, containing a flavin-binding fold conserved in flavin-dependent halogenases and monooxygenases, and an additional unique helical region at the C-terminus. This C-terminal region blocks a putative substrate-binding cleft, suggesting that a conformational change involving repositioning of this region is necessary to allow binding of the pyrrolyl-S-PltL substrate for its dichlorination by PltA.