No abstract

Myxobacteria are gram-negative soil bacteria that are well known for the production of diverse bioactive secondary metabolites [1-4]. DKxanthenes are a class of yellow pigments produced by myxobacteria belonging to the genera Myxococcus and Stigmatella (Fig. 1) [5, 6]. Myxococcus xanthus displays phase variations between yellow-and tan-phase cells [7]. DKxanthenes are produced by yellow-phase cells but not by tan-phase cells [8]. DKxanthenes are required for developmental sporulation in M. xanthus. Mutant cells of M. xanthus defective in DKxanthene biosynthesis produce severely reduced amounts of heat-resistant viable spores [5]. It has been suggested that tan-phase cells are the progenitors of spores and yellow-phase cells provide factors that tan-phase cells need to produce viable spores [9]. Later, DKxanthene was suggested as one of the factors needed by tan-phase cells for the maturation of spores in M. xanthus [5]. The majority of secondary metabolites isolated from myxobacteria are biosynthesized by polyketide synthases (PKSs) and/or non-ribosomal peptide synthases (NRPSs) [10]. DKxanthenes are also biosynthesized by PKSs and NRPSs. The structure of DKxanthenes from M. xanthus was elucidated and their biosynthetic gene clusters were identified in M. xanthus DK1622 and Stigmatella aurantiaca DW4/3-1 [5, 6]. The organization of the two biosynthetic gene clusters identified in M. xanthus DK1622 and S. aurantiaca DW4/3-1 was similar but not identical. Both clusters have core PKS and NRPS genes for the production of the DKxanthene backbone structure. However, S. aurantiaca has additional PKS genes, dkxQR and dkxT. The organization of the associated genes was also different (Fig. 2) [6]. A total of 13 DKxanthene derivatives were identified from M. xanthus DK1622, whereas S. aurantiaca DW4/3-1 was reported to produce five derivatives among them [6]. M. stipitatus is another species of the genus Myxococcus. M. stipitatus forms multicellular fruiting bodies consisting