Fig. 1.
The proposed pathway for orsellinic acid biosynthesis and modification en route to calicheamicin (5) construction. The putative reaction catalyzed by CalO2 is highlighted within the box and the mature orsellinate progeny in the context of calicheamicin is emphasized in blue. Based upon the ligand-binding studies described, `R' is anticipated to be an ACP or CoA thioester.