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Ustilago maydis is the causative agent of corn smut disease. The pathogenic dikaryotic form is generated after mating of two compatible cells. Results from several laboratories have shown that the signalling pathways required for transmission of the pheromone signal during mating are also needed during pathogenic development. In particular, the components of a MAP kinase module as well as tightly regulated cAMP signalling are needed for disease progression. Recent results suggest that these pathways have partially overlapping as well as distinct functions.
To analyse these pathways in more detail, a comparative transcriptome analysis was performed. Isogenic strains were generated which allow to activate the different signalling pathways. Wild type alleles or non-phosphorylatable alleles of the two MAP kinases kpp2 and kpp6, respectively, were introduced into strains that harbour a constitutively active allele of the MAPKK fuz7. The expression of the fuz7 allele is made inducible by the crg1 promoter which is ON in arabinose and OFF in glucose. In addition a strain was constructed that expresses the catalytic subunit of the PKA (adr1) under control of the crg1 promoter. The transcription profile of pheromone stimulated wild type cells was included for comparison.
RNA was isolated at various time points and analysed by whole genome micro arrays (Affymetrix). In this set were ten genes with a putative function in iron uptake clustered to three chromosomal regions. The cluster contains the known genes sid1 and sid2, involved in siderophore biosynthesis plus 8 new genes, designated fer1-8. All these genes are repressed by iron and require an intact cAMP signalling cascade for expression. The gene of a high affinity iron permease, fer2, was analysed in more detail and shown to be a critical virulence factor in Ustilago maydis.