Identification of a novel gene hbrB required for polarised growth in Aspergillus nidulans

We have cloned a novel gene, hbrB, by complementation of a temperature sensitive hyperbranching (hbr) mutant of Aspergillus nidulans. The mutant, hbrB3, exhibits hyperseptation and shows a marked increase in hyphal branching at the restrictive temperature. A genomic library incorporating the AMA1 sequence, which confers autonomous replication on the plasmid, was used to clone the gene. Co-ordinate loss of the complementing plasmid and wild type phenotype was shown. The 847 amino acid predicted product of the hbrB gene shows significant identities with Neurospora crassa and Aspergillus fumigatus hypothetical proteins but no other obvious relative outside the filamentous fungi. The gene was placed under control of the alcA promoter and downregulation resulted in a loss of polarity phenotype. These results suggest that A. nidulans hbrB is specific to filamentous fungi and is essential for hyphal growth and polarity. Index descriptors: Aspergillus nidulans; branching; hbrB; hyphal growth and polarity