J. Bacteriol., Dec 1996, 7265-7275, Vol 178, No. 24
Copyright © 1996, American Society for Microbiology

The Bradyrhizobium japonicum fegA gene encodes an iron-regulated outer membrane protein with similarity to hydroxamate-type siderophore receptors [published erratum appears in J Bacteriol 1997 Aug;179(16):5246]

K LeVier and ML Guerinot
Department of Biological Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA.

Iron is important in the symbiosis between soybean and its nitrogen- fixing endosymbiont Bradyrhizobium japonicum, yet little is known about rhizobial iron acquisition strategies. Analysis of outer membrane proteins (OMPs) from B. japonicum 61A152 identified three iron- regulated OMPs in the size range of several known receptors for Fe(III)- scavenging siderophores. One of the iron-regulated proteins, FegA, was purified and microsequenced, and a reverse genetics approach was used to clone a fegA-containing DNA fragment. Sequencing of this fragment revealed a single open reading frame of 750 amino acids. A putative N- terminal signal sequence of 14 amino acids which would result in a mature protein of 736 amino acids with a molecular mass of 80,851 Da was predicted. FegA shares significant amino acid similarity with several Fe(III)-siderophore receptors from gram-negative bacteria and has greater than 50% amino acid similarity and 33% amino acid identity with two [corrected] bacterial receptors for hydroxamate-type Fe(III)- siderophores. A dendrogram describing total inferred sequence similarity among 36 TonB-dependent OMPs was constructed; FegA grouped with Fe(III)-hydroxamate receptors. The transcriptional start site of fegA was mapped by primer extension analysis, and a putative Fur- binding site was found in the promoter. Primer extension and RNA slot blot analysis demonstrated that fegA was expressed only in cells grown under iron-limiting conditions. This is the first report of the cloning of a gene encoding a putative Fe(III)-siderophore receptor from nitrogen-fixing rhizobia.

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