The prosthetic group of yeast fatty acid synthase (FAS), 4'-phosphopantetheine, is covalently linked to Ser180 of subunit alpha. It originates from coenzyme A and is transferred to the enzyme by a specific phosphopantetheine:protein transferase (PPTase). The present study demonstrates that the FAS-activating PPTase of yeast represents a distinct catalytic domain of the FAS complex and resides within the C-terminal portion of subunit alpha. The autoactivation capacity of yeast FAS became evident from in vitro pantetheinylation studies using purified apo-FAS preparations. These were readily converted to pantetheinylated holo-FAS simply upon addition of free coenzyme A. Pantetheinylation-competent apo-FAS was prepared in vitro by constructing hybrid oligomers containing alpha-subunits from two different pantetheine-less FAS-mutants. The respective mutants were selected according to their ability to complement each other, in vivo. In vitro formation of hybrid apo-FAS complexes was achieved by dimethylmaleic anhydride (DMMA) -induced reversible dissociation of mixtures of the two constituent mutant enzymes. This treatment was both necessary and sufficient to produce pantetheinylation-competent apo-FAS. Specific FAS activities were comparable independent of whether the apo-enzymes were pantetheinylated in vivo or in vitro. Apart from the induction of overall FAS activity, incorporation of phosphopantetheine into apo-FAS was also demonstrated by the use of 3H-labelled coenzyme A, leading to the formation of radioactively labelled FAS. It is concluded that pantetheinylation of yeast FAS is performed by an intrinsic catalytic activity of the apo-enzyme proper. The endogenous PPTase acts in trans between different subunits alpha in the alpha6beta6 oligomer. The self-pantetheinylation of yeast FAS represents the first example of an apo-enzyme being capable of post-translational autoactivitation.