Single-chain fusions of two unrelated homeodomain proteins trigger pathogenicity in Ustilago maydis.

Pathogenic and sexual development of the fungus Ustilago maydis, the causal agent of corn smut disease, is regulated by heterodimerization of two unrelated homeodomain proteins bE and bW, both encoded by the multi-allelic b mating-type locus. This complex can only be formed if the two proteins are derived from different alleles. The heterodimer is believed to function as a transcriptional regulator that binds to target sites upstream of developmentally regulated genes. We have synthesized a translational fusion in which bE is tethered to bW by a designed flexible kink region. U. maydis strains expressing this synthetic b-fusion become pathogenic for corn illustrating that the single-chain fusion substitutes for the active bE/bW heterodimer. Synthetic b-fusions in which bE and bW originate from the same allele as well as fusions deleted for the dimerization domains were shown to be active while both homeodomains were required for function. Such active fusion proteins are expected to be instrumental in the identification of pathogenicity genes.