Endogenous circadian clocks, present in both prokaryotes and eukaryotes, provide the machinery by which organisms keep in synchrony with the external world. The ability of the clock to measure accurate time allows changes in the environment (particularly light and temperature) to be anticipated so that biological activities can occur at the appropriate times of day. The observed circadian rhythms are, as a general feature, established through the daily control of specific target genes by the clock. Demonstrations of circadian rhythms are widespread, and in the fungi the clock has been shown to control daily rhythms in spore development and liberation. Within the fungi, Neurospora crassa provides the preeminent model system for investigations into the underlying processes of circadian rhythms. Through genetic and molecular approaches, significant progress has been made in describing the N crassa circadian system. As discussed in this review, the analysis of the N. crassa clock has provided important details on (i) how the clock is assembled and (ii) how environmental signals are perceived by the clock, and (iii) a description of rhythmic output genes that are regulated by the clock. 1996 Indian Academy of Sciences.
