Regulation of the Aspergillus nidulans hisB gene by histidine starvation

The hisB gene of the filamentous fungus Aspergillus nidulans encodes imidaz ole glycerol-phosphate dehydratase (E.C. 4.2.1.19), which catalyses the sev enth enzymatic step in histidine biosynthesis. The gene was isolated and it s deduced peptide sequence of 247 amino acids showed up to 54% identity wit h the IGPD enzymes of organisms comprising all three kingdoms. Expression o f hisB cDNA in a Saccharomyces cerevisiae his3 Delta mutant strain function ally complemented the growth phenotype under histidine limitation. Addition of histidine did not affect hisB mRNA levels A. nidulans wild-type cells. Histidine starvation conditions increased the hisB transcript level four-fo ld, suggesting regulation by a cross-pathway regulatory network. Deletion o f the complete hisB open reading frame in A. nidulans strain A234 resulted in histidine auxotrophy. Additionally, hisB deletion strains were blocked f rom sexual fruiting body formation on medium containing low concentrations of histidine. This developmental phenotype of the hisB deletion mutant stra in correlated with the induction of the cross-pathway control system.