Conidial Germination in Aspergillus nidulans Requires RAS Signaling and Protein Synthesis

The dormant spores of Aspergillus nidulans become competent for growth and nuclear division in a process called conidial germination. To analyze the molecular details of conidial germination, we developed a genetic screen in which we identified spore germination-deficient mutants that are blocked in this process at the restrictive temperature. These mutants defined eight genes, of which we identified five. Four of the five were directly involved in translation and protein folding, and the fifth showed a high degree of homology to a malonyl CoA synthetase. These results suggest that out of a wide array of processes occurring during conidial germination, translation is essential if germination is to proceed. We also show that conidia containing a mutant-activated form of rasA, the ras homologue in A. nidulans, germinate in the absence of an inducing carbon source, suggesting an important role for rasA signaling in conidial germination. Together these data suggest a model by which a carbon source activates a ras-dependent sensory mechanism, inducing translation and leading to conidial germination. This study shows that conidial germination in A. nidulans requires protein synthesis and that the initiation of translation is linked, through an as yet to be determined signaling cascade that includes rasA, to a carbon-source-sensing apparatus.