This project was supported by grants of the Plan National en Biotecnologia (BI094-0037) and of the European Union (BIOTECH programme PL932067 and Human Capital and Mobility program, network project CT94-0570). F.L. was the recipient of a predoctoral grant from the FICYT, Asturias. We are especially thankful to Carmen Vilches for her help in the early stages of this project. We also wish to thank Alfredo F. Braña for his collaboration in the HPLC analysis. We are also grateful to Lilly Co. for cosmid pKC505 and to Pfizer Co. for the kind gift of mithramycin.