c-Jun and RACK1 homologues regulate a control point for sexual development in Aspergillus nidulans.

Amino acid limitation results in impaired sexual fruit body formation in filamentous fungi such as Aspergillus nidulans. The starvation signal is perceived by the cross-pathway regulatory network controlling the biosynthesis of translational precursors and results in increased expression of a transcriptional activator encoded by a c-Jun homologue. In the presence of amino acids, the gene product of the mammalian RACK1 homologue cpcB is required to repress the network. Growth under amino acid starvation conditions permits the initiation of the sexual developmental programme of the fungus, but blocks fruit body formation before completion of meiosis. Accordingly, arrest at this defined control point results in microcleistothecia filled with hyphae. Addition of amino acids results in release of the block and completion of development to mature ascospores. The same developmental block is induced by either overexpression of c-Jun homologues or deletion of the RACK1 homologue cpcB of A. nidulans in the presence of amino acids. Therefore, the amino acid starvation signal regulates sexual development through the network that also controls the amino acid biosynthetic genes. Expression of the RACK1 gene suppresses the block in development caused by a deletion of cpcB. These data illuminate a connection between metabolism and sexual development in filamentous fungi.