Preproteins targeted to the Sec-independent protein transport systems of plant thylakoids and of bacteria both have unusual transfer peptides bearing a consensus twin-arginine motif. Possible mechanistic similarity between the two Sec-independent transport pathways was investigated by assessing the ability of bacterial twin-arginine transfer peptides to direct thylakoid import. High efficiency import mas observed. This process was demonstrated to occur specifically via the Sec-independent Delta pH pathway and to depend on an intact twin-arginine motif on the transfer peptide. These results provide strong evidence for the operation of mechanistically related Sec-independent protein transport pathways in chloroplasts and bacteria. (C) 1998 Federation of European Biochemical Societies.