Novel compounds produced by Streptomyces lydicus NRRL 2433 engineered mutants altered in the biosynthesis of streptolydigin.

Streptolydigin is a tetramic acid antibiotic produced by Streptomyces lydicus NRRL 2433 and involving a hybrid polyketide synthase (PKS)-nonribosomal peptide synthetase (NRPS) system in its biosynthesis. The streptolydigin amino-acid precursor, 3-methylaspartate, has been proposed to be condensed to the polyketide portion of the molecule by a NRPS composed by three enzymes (SlgN1, SlgN2 and SlgL). On the other hand, biosynthesis of the polyketide moiety involves the participation of cytochrome P450 SlgO2 for the correct cyclization of the characteristic bicyclic ketal. Independent disruption of slgN1, slgN2, slgL or slgO2 resulted in S. lydicus mutants unable to produce the antibiotic thus confirming the involvement of these genes in the biosynthesis of the antibiotic. These mutants did not accumulate any streptolydigin biosynthesis intermediate or shunt product derived from early polyketides released from the PKS. However, they produced three novel compounds identified as 4-(2-carboxy-propylamino)-3-chloro-benzoic acid, 4-(2-carboxy-propylamino)-3-hydroxy-benzoic acid and 4-(2-carboxy-propylamino)-benzoic acid, which were designated as christolane A, christolane B and christolane C, respectively. These compounds have been shown to exert some antibiotic activity.