Submitted to: European Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/5/1996
Publication Date: N/A
Interpretive Summary: How microbes control iron uptake is essential to their longterm survival as iron is both an essential trace element while at the same time being toxic if taken up in excess quantities. Ustilago maydis, the cause of corn smut disease, has proven to be an excellent model system for studying how iron regulates its own uptake. We have identified a protein called Urbs1 which is able to turn off genes that are involved in iron uptake in the fungus. We have made progress in understanding how this protein turns genes off that are involved in iron uptake. This work has significance to our understanding of how iron may control the outcome of soilborne fungal diseases as a number of bacterial "biocontrol agents" appear to function by competing for iron with pathogenic fungi. This work may also lend insight on how the iron status of fungal spores may affect their viability and germinability.
Technical Abstract: The sid1 and urbs1 genes encode L-ornithine N5-oxygenase and a GATA family transcription regulator, respectively, for siderophore biosynthesis in Ustilago maydis. The basic promoter and iron regulatory sequences of the Ustilago maydis sid1 gene were defined by fusing restriction and BAL31 nuclease-generated deletion fragments of the promoter region with the Escherichia coli ß- glucuronidase (GUS) reporter gene. Sequences required for basal expression of sid1 mapped within 1043 bp upstream of the translation start site and include the first untranslated exon and first intron. Sequences needed for iron-regulated expression of sid1 were localized to a 306 bp region mapping 2.3 and 2.6 kb upstream of the ATG. The 306 bp region contains two G/TGATAA sequences, consensus DNA binding sites of GATA family transcription factors. Deletion or site-directed mutation of either or both GATA sequences resulted in deregulated expression of sid1. In vitro DNA binding studies showed that Urbs1 binds to the 3'-GATA site in the 306 bp iron responsive region. However, deletion of 1.1 kb between the distal GATAs and the basal promoter region led to deregulated expression of GUS, indicating that these GATA sequences are by themselves insufficient to regulate sid1. In vitro DNA binding and in vivo reporter gene analysis revealed that siderophores are not corepressors of Urbs1.