Partial repression of siderophore-mediated iron transport in Azotobacter vinelandii grown with mineral iron

Azotobacter vinelandii grown in iron-limited medium containing increased amounts of the minerals olivine or glauconite produced decreased amounts of siderophores and iron-repressible outer membrane proteins. These minerals caused a relatively rapid repression of the pyoverdin-type siderophore, azotobactin, and a slower repression of the catechol siderophores, azotochelin and aminochelin. A 77 000 molecular weight outer membrane protein also was repressed with increased mineral content of the medium, but coordinate repression with any one siderophore was not evident. Cells grown with increased mineral concentrations progressively lost siderophore-mediated iron-transport activity. This loss in activity had the greatest effect on azotobactin-mediated 55Fe uptake, but catechol siderophore mediated 55Fe uptake also was depressed. These results suggested that an additional component was required for maximal iron-transport activity promoted by all the siderophores of A. vinelandii.