The Role Of A Nonribosomal Peptide Synthetase In L-Lysine Lactamization During Capuramycin Biosynthesis

Capuramycins are one of several known classes of natural products that contain an L-Lys-derived l-alpha-amino-epsilon-caprolactam ( l-ACL) unit. The alpha-amino group of L-ACL in a capuramycin is linked to an unsaturated hexuronic acid component through an amide bond that was previously shown to originate by an ATP-independent enzymatic route. With the aid of a combined in vivo and in vitro approach, a predicted tridomain nonribosomal peptide synthetase CapU is functionally characterized here as the ATP-dependent amide-bond-forming catalyst responsible for the biosynthesis of the remaining amide bond present in L-ACL. The results are consistent with the adenylation domain of CapU as the essential catalytic component for L-Lys activation and thioesterification of the adjacent thiolation domain. However, in contrast to expectations, lactamization does not require any additional domains or proteins and is likely a nonenzymatic event. The results set the stage for examining whether a similar NRPS-mediated mechanism is employed in the biosynthesis of other L-ACL-containing natural products and, just as intriguingly, how spontaneous lactamization is avoided in the numerous NRPS-derived peptides that contain an unmodified L-Lys residue.