A Global Circadian Repressor Controls Antiphasic Expression of Metabolic Genes in Neurospora

Abstract

The white-collar complex (WCC), the core transcription factor of the circadian clock of Neurospora, activates morning-specific expression of the transcription repressor CSP1. Newly synthesized CSP1 exists in a transient complex with the corepressor RCM1/RCO1 and the ubiquitin ligase UBR1. CSP1 is rapidly hyperphosphorylated and degraded via UBR1 and its ubiquitin conjugase RAD6. Genes controlled by CSP1 are rhythmically expressed and peak in the evening (i.e., in antiphase to morning-specific genes directly controlled by WCC). Rhythmic expression of these second-tier genes depends crucially on phosphorylation and rapid turnover of CSP1, which ensures tight coupling of CSP1 abundance and function to the circadian activity of WCC. Negative feedback of CSP1 on its own transcription buffers the amplitude of CSP1-dependent oscillations against fluctuations of WCC activity. CSP1 predominantly regulates genes involved in metabolism. It controls ergosterol synthesis and fatty acid desaturases and thereby modulates the lipid composition of membranes.

Metabolic compensation of the Neurospora clock by a glucose-dependent feedback of the circadian repressor CSP1 on the core oscillator

“…CSP1 is a circadian repressor with some similarities to the yeast transcription repressors NRG1 and NRG2 (Sancar et al 2011). Since NRG1 and NRG2 are regulated by glucose (Kuchin et al 2002;Berkey et al 2004;Vyas et al 2005), we asked whether CSP1 modulates the Neurospora circadian clock in response to glucose levels.…”

“…To determine how CSP1 levels affect the circadian clock, we generated a frq-luc reporter strain that expresses CSP1-R1 Flag-His , a stable functional version of CSP1 (Sancar et al 2011), under the control of the quinic acid (QA)-inducible qa2 promoter. In the absence of QA, the qa2-csp1-R1 Flag-His strain displayed a robust conidiation rhythm on race tubes with a period of 22.18 6 0.07 h. At 60 mM QA, the period was ;2.5 h longer (24.52 6 0.45 h), and at 6 mM QA, the overt conidiation rhythm was lost ( Fig.…”

“…The data indicate that CSP1 shifts the phase of the clock under entrained conditions. Since CSP1 affects the circadian period and the phase of the clock at high but not low glucose levels, we analyzed its expression in response to glucose levels in a strain expressing csp1 Flag-His under the control of the endogenous csp1 promoter (Sancar et al 2011). Levels of csp1 Flag-His RNA and CSP1 Flag-His protein were higher in 2% than in 0.1% glucose (Fig.…”

“…Analysis of our published CSP1 chromatin immunoprecipitation (ChIP) combined with deep sequencing (ChIPseq) (Sancar et al 2011) data suggests that CSP1 binds to the wc1 promoter (Fig. 4A).…”

“…We recently showed that the WCC-controlled gene conidial separation 1 (csp1) encodes a global circadian repressor that modulates expression of ;800 genes in Neurospora (Sancar et al 2011). Many of these genes are rhythmically expressed with an evening-specific phase.…”