From 1987 onwards, a superfamily of acyl adenylate-forming enzymes, including acyl-CoA synthetases, peptide synthetases and amino acid-adding enzymes, has been identified from sequence data [ l] (Table 1). It now has been possible to relate these structurally to other acyl adenylate formers such as amino acyl-tRNA synthetases 121, and to several enzymes using phosphate intermediates in carboxyl group activation [ 3 ] . Interest in the peptide synthetase family has been motivated by the search for a ribosome-independent path to peptides, since multienzymes have been shown to catalyse up to 20 consecutive condensing steps [4]. Peptides originating from the multienzyme pathway have a considerably expanded structural spectrum including, not only I)-amino acids, but also many unusual compounds together with hydroxy acids [ S ] .