Figure 1. Tan strains activate pathways dedicated to acquisition of iron.

Many of the genes whose expression is increased significantly in the tan strains comprise the siderophore production and transport, hemin transport, and proteins that provide energy for transport of iron-binding compounds. A partial list of components produced in tan cells is shown in this cartoon. The blue/purple circles represent the siderophore myxochelin pathway (MXAN_3639-3647) [39], [40]. Myxochelin (mxc) is shown as a yellow star that binds Fe³⁺ when exported. Mxc•Fe³⁺ may be imported by a FepA-like protein (candidate is MXAN_6911; dark green shape), located in the outer membrane [41]. Light green shapes represent the ferric siderophore ABC transporter genes (permease and MXAN_0684-0687 in membrane); brown cylinders represent periplasmic energy transduction TonB proteins (MXAN_0276, MXAN0820, and 6485) that may energize the outer membrane proteins (dashed lines). Red/orange icons represent hemin transporters (MXAN_1318-1321). Not shown are RhtX/FptX siderophore transporter (MXAN_5357), iron ABC transporters (MXAN_0770-0772), iron compound ABC transporter, and a periplasmic iron compound binding protein, Fe•ABC (MXAN_6000). Search Tool for the Retrieval of Interacting Genes/Proteins (STRING) software [42] predicts MXAN_3639 (putative iron-chelator utilization protein in the myxochelin operon) interacts with the ferric siderophore ABC transporter permease (MXAN_0685, 0686).