Generation of D amino acid residues in assembly of arthrofactin by dual condensation/epimerization domains

The first 6 residues of the biosurfactant lipopeptidolactone arthrofactin have the D configuration, yet none of the 11 modules of the nonribosomal peptide synthetase assembly line have epimerization domains. We show that the two-module ArfA subunit and the first module of the ArfB subunit, which act in tandem to produce the N-acyl-D-Leu(1)-D-AsP2-DThr(3)-S-protein intermediate, activate the L amino acids and epimerize them as the aminoacyl-S-pantetheinyl T domain intermediates before the next downstream condensation. The condensation (C) domains are shown to have C-D(L) chirality in peptide bond formation. The upstream aminoacyl/peptidyl moiety is epimerized before condensation only when the condensation domains are simultaneously presented with the L-aminoacyl-S-pantetheinyl acceptor. These D CL catalysts are dual function condensation/epimerization domains that can be predicted by bloinformatics analysis to be responsible for incorporation of all D residues in arthrofactin and of D residues in syringomycin, syringopeptin, and ramoplanin synthetases.