Insight into the structure–function relationship of the nonheme iron halogenases involved in the biosynthesis of 4‐chlorothreonine – T hr3 from S treptomyces sp. OH ‐5093 and S yr B 2 from P seudomonas syringae pv. syringae B 301 DR

Molecular cloning of the biosynthetic gene cluster involved in the production of free 4‐chlorothreonine in S treptomyces sp. OH ‐5093 showed the presence of six ORF s: thr1 , thr2 , thr3 , orf1 , orf2 and thr4 . According to bioinformatic analysis, thr1 , thr2 , thr3 and thr4 encode a free‐standing adenylation domain, a carrier protein, an F e(II) nonheme α‐ketoglutarate‐dependent halogenase and a thioesterase, respectively, indicating the role of these genes in the activation and halogenation of threonine and the release of 4‐chlorothreonine in a pathway closely reflecting the formation of this amino acid in the biosynthesis of the lipodepsipeptide syringomycin from P seudomonas syringae pv. syringae B 301 DR . Orf1 and orf2 show sequence similarity with alanyl/threonyl‐t RNA synthetases editing domains and drug metabolite transporters, respectively. We show that thr3 can replace the halogenase gene syrB2 in the biosynthesis of syringomycin, by functional complementation of the mutant P . s . pv.  syringae strain BR 135 A 1 inactivated in syrB2 . We also provide an insight into the structure–function relationship of halogenases T hr3 and S yr B 2 using homology modelling and site‐directed mutagenesis. Database Nucleotide sequence data have been deposited in the ebi.ac.uk/ EMBL /GenBank databases under accession numbers: thr1 (gi:378781338| CCF23454 ); thr2 (gi:378781340| CCF23456 ); thr3 (gi:378781341| CCF23457 ); orf1 (gi:378781342| CCF23458 ); orf2 (gi:378781343| CCF23459 ); thr4 (gi:378781344| CCF23460 )