1887

Abstract

forms both catecholate and azotobactin siderophores during iron-limited growth. Azotobactin is repressed by about 3 μM iron, but catecholate siderophore synthesis continues up to a maximum of 10 μM iron. This suggests that catecholate siderophore synthesis is regulated by other factors in addition to the ferric uptake repressor (Fur). In this study the first gene required for catecholate siderophore biosynthesis, which encodes an isochorismate synthase (), was isolated. The region upstream of contained a typical σ promoter, with an iron-box overlapping the −35 sequence and a Sox-box (Box 1) overlapping the −10 sequence. Another Sox-box was found further upstream of the −35 sequence (Box 2). Also upstream, an unidentified gene () was detected which would be transcribed from a divergent promoter, also controlled by an iron-box. The activity of and a :: fusion was negatively regulated by iron availability and upregulated by increased aeration and by superoxide stress. The iron-box in the promoter was 74% identical to the Fur-binding consensus sequence and bound the Fur protein of with relatively high affinity. Both Box 1 and Box 2 were in good agreement with the consensus sequence for binding the SoxS protein of and Box 1 was in very good agreement with the Sox-box found in the promoter of , which is also regulated by superoxide stress. Both Sox-boxes bound a protein found in cell extracts, with Box 1 exhibiting the higher binding affinity. The Sox protein identified in this assay appeared to be constitutive, rather than inducible by superoxide stress. This indicates that the Sox response in is different from that in . These data support the hypothesis that catecholate siderophore biosynthesis is under dual control, repressed by a Fur–iron complex and activated by another DNA-binding protein in response to superoxide stress. The interaction between these regulators is likely to account for the delay in ferric repression of catecholate siderophore production, since these siderophores have an additional role to play in the protection of iron-limited cells against oxidative damage.

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2000-07-01
2024-03-28
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