Abstract
Pactamycin is a potent antitumor antibiotic produced by the soil bacterium Streptomyces pactum. This structurally unique natural product consists of a highly functionalized cyclopentitol core unit, two aromatic rings [3-ami-noacetophenone (3АAР) and 6-methylsalicylic acid (6MSA)], and a 1,1-di-methylurea moiety. Despite its strong biological activity, its development was hampered by its high-toxicity profile. Our previous studies revealed that the type I iterative PKS (PtmQ) is a 6MSA synthase that supplies 6MSA for pactamycin biosynthesis. However, the enzyme that is responsible for the attachment of 6MSA to the aminocyclitol unit was unknown. Through genetic and biochemical characterization, we discovered that PtmR, a ß-ketoacyl-ACP synthase (KAS) III-like protein, is responsible for the di-rect transfer of the 6-methylsalicylyl moiety from PtmQ to the aminocy-clopentitol unit. The enzyme also recognizes a wide array of synthetically prepared acyl-N-acetylcysteamines (acyl-NACs) as substrates.