Dual regulation of catecholate siderophore biosynthesis in Azotobacter vinelandii by iron and oxidative stress

Azotokacter vinelandii forms both catechotate and azotobactin siderophores during iron-limited growth. Azotobactin is repressed by about 3 mu M iron, but catecholate siderophore synthesis continues up to a maximum of 10 mu M iron. This suggests that catecholate siderophore synthesis is regulated by other factors in addition to the ferric uptake repressor (Fur), In this stu dy the first gene required for catecholate siderophore biosynthesis, which encodes an isochorismate synthase (csbC), was isolated. The region upstream of csbC contained a typical sigma(70) promoter, with an iron-box overlappi ng the -35 sequence and a Sox-box (Box 1) overlapping the -10 sequence. Ano ther Sox-box was found further upstream of the -35 sequence (Box 2), Also u pstream, an unidentified gene (orfA) was detected which would be transcribe d from a divergent promoter, also controlled by an iron-box, The activity o f csbC and a csbC::luxAB fusion was negatively regulated by iron availabili ty and upregulated by increased aeration and by superoxide stress. The iron -box in the csbC promoter was 74% identical to the Fur-binding consensus se quence and bound the Fur protein of Escherichia coli with relatively high a ffinity. Both Box I and Box 2 were in flood agreement with the consensus se quence for binding the SoxS protein of E. coil and Box 1 was in very good a greement with the Sox-box found in the fpr promoter of A. vinelandii, which is also regulated by superoxide stress, Both Sox-boxes bound a protein fou nd in A, vinelandii cell extracts, with Box 1 exhibiting the higher binding affinity. The Sox protein identified in this assay appeared to be constitu tive, rather than inducible by superoxide stress. This indicates that the S ox response in A. vinelandii is different from that in E. coli. These data support the hypothesis that catecholate siderophore biosynthesis is under d ual control, repressed by a Fur-iron complex and activated by another DNA-b inding protein in response to superoxide stress. The interaction between th ese regulators is likely to account for the delay in ferric repression of c atecholate siderophore production, since these siderophores have an additio nal role to play in the protection of iron-limited cells against oxidative damage.