Catalyzes the release of the completed linear polyketide from the rif PKS by forming an intramolecular amide bond, in this way terminating polyketide assembly and forming the macrocyclic compound proansamycin X, an intermediate in the rifamycin B biosynthesis. Antibiotic biosynthesis; rifamycin B biosynthesis. Inactivation of rifF gives a rifamycin B non-producing mutant that still accumulates a series of linear polyketides ranging from the tetra- to a decaketide, also detected in the wild-type. Belongs to the arylamine N-acetyltransferase family.