A protein-synthesizing system based on a minimal set of purified components was used to investigate the roles molecular chaperones play in the folding of newly synthesized polypeptides. After we ascertained that this system lacks intrinsic chaperones, the effect of adding chaperones in a co-translational or post-translational manner was directly evaluated. An aggregation-prone single-chain antibody was used as the model nascent chain. The participation of the trigger factor or the DnaK system during translation efficiently increased the level of functional protein that was generated. In addition, both systems also acted as chaperones after translation had been stopped. In contrast, the GroEL/ES system showed little or no co- or post-translational assistance in folding. (C) 2004 Published by Elsevier Inc.