c-Jun and RACK1 homologues regulate a control point for sexual developmentin Aspergillus nidulans

Amino acid limitation results in impaired sexual fruit body formation in fi lamentous fungi such as Aspergillus nidulans. The starvation signal is perc eived by the cross-pathway regulatory network controlling the biosynthesis of translational precursors and results in increased expression of a transc riptional activator encoded by a c-Jun homologue. In the presence of amino acids, the gene product of the mammalian RACK1 homologue cpcB is required t o repress the network. Growth under amino acid starvation conditions permit s the initiation of the sexual developmental programme of the fungus, but b locks fruit body formation before completion of meiosis. Accordingly, arres t at this defined control point results in microcleistothecia filled with h yphae. Addition of amino acids results in release of the block and completi on of development to mature ascospores. The same developmental block is ind uced by either overexpression of c-Jun homologues or deletion of the RACK1 homologue cpcB of A. nidulans in the presence of amino acids. Therefore, th e amino acid starvation signal regulates sexual development through the net work that also controls the amino acid biosynthetic genes. Expression of th e RACK1 gene suppresses the block in development caused by a deletion of cp cB. These data illuminate a connection between metabolism and sexual develo pment in filamentous fungi.