Day/night and circadian rhythm control of con gene expression in Neurospora

In the filamentous fungus Neurospora crassa, several events in the process of conidiation are influenced by light. Two genes, con-6 and con-10, which were previously shown to be transcriptionally activated during conidiation and by exposure to light, were found to be unexpressed in mycelium maintained in constant darkness or in constant light. However, when mycelium was shifted from darkness to light, transcripts of both genes appeared and were abundant. Upon further illumination both transcripts disappeared--i.e., their continued production was light repressed. When dark-grown mycelium was exposed to a light pulse and reincubated in the dark, expression of con-6 and con-10 exhibited a 20-hr circadian periodicity. Both genes were photoinducible throughout the stages of the circadian cycle. In the mutant strains bd and bd;frq9, con-6 and con-10 were light inducible but were not normally light repressible. Mutant genes such as acon-2, acon-3, and fl that block developmental expression of con-6 and/or con-10 did not prevent their photoinduction.