SEQUENCES AND PROTEINS REQUIRED FOR IRON-REGULATED EXPRESSION OF SID1OF USTILAGO-MAYDIS

The molecular biology of the high affinity, siderophore-mediated iron uptake system of the basidiomycete fungus Ustilago maydis is under inv estigation. Ustilago maydis produces two cyclic peptide siderophores, ferrichrome and ferrichrome A. Biosynthesis of both siderophores is in itiated by ornithine-N-5-oxygenase, the product of sid1. sid1 mRNA acc umulates only during growth under iron starvation conditions in wild-t ype cells or constitutively in urbs1 mutants. urbs1 encodes a 100-kDa protein with putative Zn finger domains that share sequence identity w ith those of the GATA family of transcription factors. The promoter re gion of sid1 was defined by deletion analysis of a 3.0-kb region 5' to the translational start of sid1 using the Escherichia coli GUS gene a s a reporter. Three regions were defined by this analysis to be critic al to expression of sid1. These include (i) a 306-bp region containing two GATA sequences and mapping 2.4 kb from the start of translation; (ii) a 439-bp region immediately 5' to the start of transcription; and (iii) a region encompassing the first intron of sid1. Deletion of the GATA sequences resulted in deregulated expression of sid1, while elim ination of the latter two sequences ablated expression of the gene und er all circumstances. Current efforts are focused on determining wheth er Urbs1 interacts directly with the sid1 promoter via the GATA sequen ces and whether this interaction is dependent upon iron.