Identification of a target gene for the bE-bW homeodomain protein complex in Ustilago maydis

In the phytopathogenic fungus Ustilago maydis, sexual and pathogenic develo pment are controlled by the multiallelic b mating type locus. The b locus e ncodes a pair of unrelated homeodomain proteins termed bE and bW that form a heterodimeric complex when both proteins originate from different alleles . The heterodimer is presumed to be the central regulator for pathogenicity genes. Here, we show that a translational fusion protein comprising specif ic domains of bE1 and bW2 remains biologically active and binds to a sequen ce motif in the promoter of Iga2, a gene located in the a mating type locus . This b binding sequence 1 (bbs1) is also recognized by the native bE1-bW2 heterodimer in vivo and mediates the b-dependent regulation of the Iga2 ge ne. Our data demonstrate that the bE-bW heterodimer can act as a positive t ranscriptional regulator.