A Novel Cryptochrome-Dependent Oscillator in Neurospora crassa

Circadian clocks are composed of molecular oscillators and are found in most eukaryotes and some prokaryotes. The fungus Neurospora crassa is a leading model for studying the clock. In N. crassa, the well-described FRQ/WCC Oscillator (FWO) consists of a molecular feedback loop involving the negative element FRQ, the blue-light photoreceptor WC-1, and WC-2. WC-1 and WC-2 form a complex called WCC, which functions as the positive element in the feedback loop. The FWO is considered to be the core oscillator regulating overt rhythmicity. However, several labs have shown that rhythms can persist in the absence of a functional FWO under certain growth conditions and genetic backgrounds, suggesting the presence of additional oscillators in the cell. Using genetic approaches to identify components of these putative oscillators, we uncovered a mutant strain, called light-mutant 1 (LM1) that is robustly rhythmic in constant light, and in strains carrying deletions of FWO components; both of which are conditions that abolish FWO function. The oscillator (called the Light Mutant Oscillator, LMO) revealed in the LM1 mutant strain meets two of the three criteria for a circadian oscillator. The LMO has a free running period of ~ 24h, and it is temperature-compensated. However, while the LMO can respond to light cues, WC-1 is required for circadian entrainment to 24-h light cycles. The response of LM1 cells lacking the circadian blue-light photoreceptor WC-1 to blue-light suggested that alternate light inputs function in LM1 mutant cells. I show that the blue light photoreceptors VIVID and CRY compensate for each other, and for WC-1, in LMO light responses. Importantly, I show that deletion of the cryptochrome (cry) gene abolishes rhythmicity in an LM1 strain, providing evidence for a role for CRY in the clock mechanism. The LM1 mutation is recessive, suggesting loss of function. Therefore, we hypothesize that the LM1 gene encodes a protein that negatively regulates the activity of the LMO. Our mapping and sequencing data have placed the LM1 mutation on the left arm of chromosome I, near the mating type locus; however, the identity of the mutated gene remains elusive.