Saframycin M x l is a DNA-binding antibiotic and antitumour agent produced by Myxococcus xanthus. It is a heterocyclic quinone, thought to be synthesized via the linear pepide intermediate AlaGlyTyrTyr. Analysis of 14-1 kb DNA sequence involved in saframycin production revealed genes for two large multifunctional peptide synthetases of 1770 and 2605 amino acids, respectively, and a putative 0-methyltransferase of 220 amino acids. The three ORFs read in the same direction and are separated by short non-translated gaps of 4 4 and 49 bp. The peptide synthetases contain two amino-acid-activating domains each. The first domain lacks two of the most conserved 'core' sequences, and the last domain is followed by a putative reductase functionality, not previously seen in peptide synthetases. Complementation tests showed that antibiotic-nonproducing mutant strains lacking one of the peptide synthetases secrete a substrate, presumably a modified amino acid precursor, that can be used by 0-methyltransferase-def icient mutant strains to synthesize saframycin M x l .

The gene cluster for the biosynthesis of the heterocyclic quinone antibiotic saframycin Mxl of Myxococcus xanthus DM5W15 was inactivated and tagged by TnS insertions. The tagged genes were cloned in Escherichia coli and used to select overlapping cosmid clones spanning 58 kb of the M. xanthus genome.Gene disruption experiments defined a 2 18 kb contiguous DNA region involved in saf ramycin biosynthesis. Sequencing of part of this region revealed a large ORF containing two 600-amino-acid domains with similarity to peptide synthetase aminolacid-activating sequences, suggesting that saframycin M x l is synthesized by a nonribosomal multienzyme complex, similar to other bioactive peptides.