Lewis, Zachary Austin (2005-02). Control of rhythmic output from the circadian clock in Neurospora crassa. Doctoral Dissertation.
Circadian rhythms are visible as daily oscillations in biochemical, physiological, or behavioral processes. These rhythms are produced by an endogenous clock that maintains synchrony with the external environment through responses to external stimuli such as light or temperature. The clock, in turn, coordinates internal processes in a time-dependent fashion. Genetic and molecular analysis of the filamentous fungus Neurospora crassa has demonstrated that the products of the frequency (frq) and white-collar (wc-1 and wc-2) genes interact to form an interlocked feedback loop that lies at the heart of the clock in this fungus. This feedback loop, termed the FRQ/WC oscillator, produces a ~24h oscillation in frq mRNA, FRQ protein, and WC-1 protein. In turn, the FRQ/WC oscillator regulates rhythmic behavior and gene expression. The goal of this dissertation is to understand how rhythmic outputs are regulated by the FRQ/WC oscillator in Neurospora. To this end, we have taken a microarray approach to first determine the extent of clock-controlled gene expression in Neurospora. Here, we show that circadian regulation of gene expression is widespread; 145 genes, representing 20% of the genes we analyzed, are clock-controlled. We show that clockregulation is complex; clock-controlled genes peak at all phases of the circadian cycle. Furthermore, we demonstrate the clock regulates diverse biological processes, such as intermediary metabolism, translation, sexual development and asexual development. WC-1 is required for all light- and clock-regulated gene expression in Neurospora. We have shown that overexpression of WC-1 is sufficient to activate clock-controlled gene expression, but is not sufficient to induce all light-regulated genes in Neurospora. This result indicates that cycling of WC-1 is sufficient to regulate rhythmic expression of a subset of clockcontrolled genes. Conversely, a post-translational mechanism underlies WC-1 mediated light signal transduction in Neurospora. Finally, we have demonstrated the Neurospora circadian system is comprised of mutually coupled oscillators that interact to regulate output gene expression in the fungus.