Two structurally related polyene macrolides are produced by Streptomyces diastaticus var. 108: rimocidin (3a) and CE-108 (2a). Both bioactive metabolites are biosynthesized from the same pathway through type I polyketide synthases by choosing a starter unit either acetate or butyrate, resulting in 2a or 3a formation, respectively. Two additional polyene amides, CE-108B (2b) and rimocidin B (3b), are also produced "in vivo" when this strain was genetically modified by transformation with engineered SCP2*-derived vectors carrying the ermE gene. The two polyene amides, 2b and 3b, showed improved pharmacological properties, and are generated by a tailoring activity involved in the conversion of the exocyclic carboxylic group of 2a and 3a into their amide derivatives. The improvement on some biological properties of the resulting polyenes, compared with that of the parental compounds, encourages our interest for isolating the tailoring gene responsible for the polyene carboxamide biosynthesis, aimed to use it as tool for generating new bioactive compounds. In this work, we describe the isolation from S. diastaticus var. 108 the corresponding gene, pcsA, encoding a polyene carboxamide synthase, belonging to the Class II glutamine amidotransferases and responsible for "in vivo" and "in vitro" formation of CE-108B (2b) and rimocidin B (3b). The fermentation broth from S. diastaticus var. 108 engineered with the appropriate pcsA gene construction, showed the polyene amides to be the major bioactive compounds.