Biosynthesis of clorobiocin: investigation of the transfer and methylation of the pyrrolyl-2-carboxyl moiety
The 5-methyl-2-pyrrolylcarbonyl moiety of the aminocoumarin antibiotics clorobiocin and coumermycin A1 is the key pharmacophore for targeting the ATP-binding site of GyrB for inhibition of the bacterial type-II topoisomerase DNA gyrase. During the late stage of clorobiocin and coumermycin A1 biosynthesis, the pyrrolyl-2-carboxyl group is transferred from the peptidyl carrier proteins Clo/CouN1 to the 3'-hydroxyl of the 4-methoxy-L-noviosyl scaffold by the action of the acyltransferases Clo/CouN7. CouN1 and CouN7 have now been heterologously expressed and purified from Escherichia coli. The apo form of CouN1 is converted to the acyl-holo form by loading with pyrrolyl-2-carboxyl-S-pantetheinyl moieties from synthetic pyrrolyl- and 5-methylpyrrolyl-CoAs by the action of the phosphopantetheinyl transferase Sfp. CouN7 acts as an acyltransferase, moving the pyrrole acyl moieties from CouN1 to the noviose sugar of descarbamoylnovobiocin. When the 5-methylpyrrolyl-2-carboxyl-thioester of CouN1 is the cosubstrate, the in vitro product differs from clorobiocin only in a CH3 for Cl group change on the coumarin ring. Double transfer of this acyl moiety by CouN7 to the penultimate intermediate in coumermycin A1 assembly completes that antibiotic biosynthetic pathway.