New antibiotics produced by Streptomyces echinatus A8331 cultured in the presence of heterocyclic aromatic acids can be separated and purified by high-performance liquid chromatography using reversed phase columns. Natural quinoxaline antibiotics and certain quinoline derivatives can also be efficiently separated in normal phase systems. Details of purification procedures are described together with experiments to characterise the new antibiotics by field desorption mass spectrometry and proton magnetic resonance. Mono- and bis-substituted derivatives of echinomycin containing the following replacement chromophores have been isolated: 7-chloroquinoxaline-2-carbonyl, thieno[3,2-b]pyridine-5-carbonyl, and 6-methylquinoline-2-carbonyl. With a 6-methylquinoline-2-carboxylic acid precursor the analogues containing one or two replacement chromophores are each separable into two distinct components. One of the bis-substituted 6-methylquinoline products appears inactive in an antibacterial assay and behaves as a triostin analogue, presumably an immediate precursor of the corresponding echinomycin derivative.
Improved precursor-directed biosynthesis in E. coli via directed evolution