Production of Outer-membrane Proteins and an Extracellular Fluorescent Compound by Iron-limited Azomonas macrocytogenes

Abstract

Outer membranes were isolated from iron-limited and iron-sufficient Azomonas macrocytogenes ATCC 12334 by sucrose density centrifugation or treatment with Sarkosyl. Analysis of the outer membranes by sodium dodecyl sulphate polyacrylamide gel electrophoresis revealed that iron-limited cells produced greater amounts of two proteins of apparent molecular mass 74 kDa and 70 kDa than iron-sufficient cells. The enhanced production of these two proteins was evident when media contained less than 9·7 μm added iron with maximum production occurring when media contained less than 1·1 μm added iron. Iron-limited growth conditions also caused A. macrocytogenes ATCC 12334 to excrete a visible yellow, blue-white fluorescent compound into culture supernatant fluids with maximum levels being produced in media that contained less than 1·1 μm added iron. This fluorescent compound had a distinctive pH-dependent absorption maximum characteristic of pyoverdin-type siderophores and appeared to bind iron. Furthermore, culture supernatant fluids containing the fluorescent compound promoted the uptake of 55Fe in A. macrocytogenes ATCC 12334 cells. Three other A. macrocytogenes strains, NCIB 10958, NCIB 8700 and NCIB 9129, produced iron-repressible outer-membrane proteins but only the former two strains produced the iron-repressible fluorescent compound. It is proposed that the iron-regulated proteins and fluorescent compound produced by A. macrocytogenes may function in high-affinity iron uptake.