Comparative Analysis of a Putative HLH Transcription Factor Responsible for Conidiation in Aspergillus Species

Asexual reproduction or conidiation in aspergilli is a primary mean to produce their progenies that is environmentally and genetically controlled tightly. Previously, intensive researches in the model fungus Aspergillus nidulans disclosed some genes playing important roles in asexual and sexual development. Among them, one gene encoding a putative helix-loop-helix (HLH) transcription factor, named ndrA, has been isolated and characterized as a downstream regulator of developmental master regulator NsdD. By using comparative genome search of A. niduans NdrA protein, its orthologues have been identified in A. fumigatus and A. flavus, respectively (AfudrnA and AfldrnA). Deletion of the ndrA genes in both Aspergillus species made them unable to produce the conidia yet abundant production of sclerotia in A. flavus. Complementation of ndrA deletion strains by intact ndrA ORFs has restored the conidiation as in the control strains. In A. fumigatus, ndrA deletion also resulted in loss of conidiation phenotype. Northern analyses showed that the ndrA genes in both Aspergillus species are highly expressed at the early stage of the conidiation. Interestingly, the ndrA genes were found to be necessary for the proper expression of brlA genes. Antifungal sensitivity test revealed that the ndrA genes might be responsible for the sensitivity or resistance to some antifungal agents. However, ndrA deletion did not greatly influence the growth in both strains. And the A. flavus ndrA gene did not affect the aflatoxin production. Taken together, ndrA genes in Aspergillus species could be an important positive regulator of conidiation under the regulation of the nsdD gene yet upstream of the brlA gene.