URBS1, A GENE REGULATING SIDEROPHORE BIOSYNTHESIS IN USTILAGO-MAYDIS,ENCODES A PROTEIN SIMILAR TO THE ERYTHROID TRANSCRIPTION FACTOR GATA-1

Ustilago maydis secretes ferrichrome-type siderophores, ferric-ion-bin ding compounds, in response to iron starvation. TA2701, a non-enteroba ctin-producing, non-ferrichrome-utilizing mutant of Salmonella typhimu rium LT-2, was employed as a biological indicator in a novel screening method to isolate three N-methyl-N'-nitro-N-nitrosoguanidine-induced U. maydis mutants defective in the regulation of ferrichrome-type side rophore biosynthesis. These mutants displayed a constitutive phenotype ; they produced siderophores in the presence of iron concentrations th at would typically repress siderophore synthesis in wild-type strains. A 4.8-kb fragment of U. maydis genomic DNA capable of restoring norma l regulation of siderophore biosynthesis in the constitutive mutants w as identified. This segment of DNA contains an intronless open reading frame that specifies a protein of 950 amino acids containing two fing er motifs similar to those found in the erythroid transcription factor GATA-1. Disruption of this open reading frame in a wild-type strain g ave rise to cells that produced siderophores constitutively. Genetic s tudies indicated that the disruption mutation was allelic to the chemi cally induced mutations, confirming that the structural gene for a reg ulator rather than a suppressor gene had been cloned. Northern (RNA) a nalysis of the gene revealed a 4.2-kb transcript that is expressed con stitutively at low levels in wild-type cells. The data support the hyp othesis that this gene, which we designate urbs1 (Ustilago regulator o f biosynthesis of siderophores), acts directly or indirectly to repres s biosynthesis of siderophores in U. maydis.