The results of this study demonstrate that iron availability plays a role in phase variation of Myxococcus xanthus. First, comparison of otherwise isogenic yellow and tan variants revealed that tan variants up-regulate a core set of genes encoding proteins critical for acquisition of iron, including enzymes for synthesis of the siderophore myxochelin and import of FeNmyxochelin and hemin. Biochemical assays confirmed the increase in siderophore in tan variants. Second, addition of iron dramatically inhibited expression of mxc genes and slowed the yellow-to-tan Apoptosis Activator 2 switch significantly, effectively prolonging cells in the yellow phase. When the concentration of available iron was reduced, the number of tan variants arising from a wild-type yellow culture increased. Iron acquisition genes were not the only genes responsive to phase variation. As shown in Fig 7, expression of PV markers, including DKX, antibiotic myxovirescin, and kinase-like proteins decreased while expression of potential regulators, serine threonine kinases and HTH-Xre proteins, increased in WT-T. Changes in Mxv and DKX may reflect a metabolic trade-off as cells cease to invest in secondary metabolites when there is shortage of iron. Hence, loss of pigment or disruption of the pigment biosynthetic pathway alone is sufficient to activate the iron acquisition scheme in M. xanthus. The DKX pathway does not autoregulate because disruption of the dkxG gene did not Domiphen Bromide affect expression of genes in other dkx operons. Loss of DKX did not affect production of Mxv or PKc-like proteins, however.Surprisingly, although swarming is also diminished in tan variants compared with yellow variants, we did not find differences in expression of known gliding genes. General characteristics of yellow and tan variants are summarized in Fig 8A. To begin to understand the context for these PV markers, three strains that are tan due to mutation were analyzed alongside the WT-T. The dkx mutant, which is tan due to disruption of the pigment biosynthetic pathway, resembled the WT-T expression profile for mxc, hemin, and macrolide efflux genes.