The wild-type Streptomyces lividans 66 genome contains a 4 3 kb amplifiable DNA unit (AUD), and its four ORFs encode proteins that could not be identified by sequence comparison with databases. One of the gene products (encoded by orf-2) was purified and determined to be a novel 23 kDa protein. This protein is synthesized by the wild-type strain, absent in a variant lacking the AUD and overproduced in a variant in which the AUD is amplified (ADS). Immunological studies and analyses by confocal laser microscopy showed that the 23 kDa protein is associated with the substrate hyphae of the wild-type and the ADS-containing variant. Examination by microscopy revealed that the strain carrying the ADS forms bulges within the substrate hyphae and apical vesicles. These bulges have high levels of associated 23 kDa protein and contain storage-I i ke material.