Conserved across different phyla. We show that GSH undergoes circadian fluctuations in Drosophila heads, reaching its highest levels in the morning. While diurnal GSH variations were previously reported in different mammalian organs, such as the liver [42], the underlying molecular Mirin site mechanism was not elucidated. A critical finding of our study is that the generation of the GSH rhythm in Drosophila heads involves transcriptional regulation of genes that encode subunits comprising GCL, the first and rate limiting enzyme 
in glutathione production. Daily rhythms for both Gclm and Gclc mRNA were discerned in LD with peak expression in the early and late night, respectively. However, Gclc mRNA did not show significant fluctuations in DD, suggesting that the rhythm may have dampened or is modulated by LD. On the other hand, the expression of both genes was significantly altered in mutants with defects in the positive or negative arm of the clock loop. Namely, expression of Gclc and Gclm 
was lower at the expected peak in cyc01 flies, which have a disrupted CLK/CYC complex, and higher at the expected trough in per01 mutants lacking periodic repression of CLK/CYC activity. Thus, our functional genetic data suggest that Gclc and Gclm may be activated by the CLK/CYC complex. ThisCircadian Control of Glutathione HomeostasisFigure 7. Circadian regulation of GstD1 expression. (A) A circadian rhythm in GstD1 mRNA levels was detected in wild type (CS) flies with a peak at ZT 8 significantly different from the trough at ZT 20 (p,0.01). (B) No significant difference was observed between ZT 8 and ZT 20 in per01 and cyc01 flies while the difference was observed in CS heads (p,0.01). Data represent average values (6 SEM) obtained from 3 independent bio-replicates and normalized to ZT 0. Data were analyzed by a 2-way ANOVA and Bonferroni’s post-tests. Different subscript letters indicate significant difference between treatment groups. doi:10.1371/journal.pone.0050454.gconclusion is consistent with a recent genome-wide study suggesting that CLK/CYC binds chromatin in the vicinity of the Gclc and Gclm gene promoters in a time dependent manner [7]. Since CLK binding could not be unambiguously mapped because of its occurrence near transcription start sites of genes adjacent to Gclc and Gclm [7], we investigated the expression of these neighboring genes and found them to be non-rhythmic. Because GSH biosynthesis is critical for cellular health, transcriptional regulation of Gclc and Gclm have been studied intensively 1326631 in mammals [19]. These genes are known to be induced by oxidative stress and electrophiles through the binding of stress 223488-57-1 web responsive transcription factors to AP-1 and electrophile response elements [43,44]. Analysis of DNA regulatory regions revealed the presence of such consensus motifs in the Drosophila Gclc and Gclm promoters (S. Radyuk, unpublished). In mammals, Gclc is induced via Keap1/Nrf2 signaling; thus we examined the transcriptional profiles of cncC, (a Drosophila homologue of mammalian Nrf2 gene), and Keap1. We did not detect a circadian rhythm for either cncC or Keap1 mRNAs, nor was there any effect of per or cyc mutations on their mRNA expression levels. However, it remains possible that post-transcriptional modification of thesefactors could be involved in the temporal modulation of Gclc and Gclm expression. In contrast to the robust rhythmic expression of Gclc and Gclm mRNAs, the protein levels of GCLc did not appear rhythmic, while var.Conserved across different phyla. We show that GSH undergoes circadian fluctuations in Drosophila heads, reaching its highest levels in the morning. While diurnal GSH variations were previously reported in different mammalian organs, such as the liver [42], the underlying molecular mechanism was not elucidated. A critical finding of our study is that the generation of the GSH rhythm in Drosophila heads involves transcriptional regulation of genes that encode subunits comprising GCL, the first and rate limiting enzyme in glutathione production. Daily rhythms for both Gclm and Gclc mRNA were discerned in LD with peak expression in the early and late night, respectively. However, Gclc mRNA did not show significant fluctuations in DD, suggesting that the rhythm may have dampened or is modulated by LD. On the other hand, the expression of both genes was significantly altered in mutants with defects in the positive or negative arm of the clock loop. Namely, expression of Gclc and Gclm was lower at the expected peak in cyc01 flies, which have a disrupted CLK/CYC complex, and higher at the expected trough in per01 mutants lacking periodic repression of CLK/CYC activity. Thus, our functional genetic data suggest that Gclc and Gclm may be activated by the CLK/CYC complex. ThisCircadian Control of Glutathione HomeostasisFigure 7. Circadian regulation of GstD1 expression. (A) A circadian rhythm in GstD1 mRNA levels was detected in wild type (CS) flies with a peak at ZT 8 significantly different from the trough at ZT 20 (p,0.01). (B) No significant difference was observed between ZT 8 and ZT 20 in per01 and cyc01 flies while the difference was observed in CS heads (p,0.01). Data represent average values (6 SEM) obtained from 3 independent bio-replicates and normalized to ZT 0. Data were analyzed by a 2-way ANOVA and Bonferroni’s post-tests. Different subscript letters indicate significant difference between treatment groups. doi:10.1371/journal.pone.0050454.gconclusion is consistent with a recent genome-wide study suggesting that CLK/CYC binds chromatin in the vicinity of the Gclc and Gclm gene promoters in a time dependent manner [7]. Since CLK binding could not be unambiguously mapped because of its occurrence near transcription start sites of genes adjacent to Gclc and Gclm [7], we investigated the expression of these neighboring genes and found them to be non-rhythmic. Because GSH biosynthesis is critical for cellular health, transcriptional regulation of Gclc and Gclm have been studied intensively 1326631 in mammals [19]. These genes are known to be induced by oxidative stress and electrophiles through the binding of stress responsive transcription factors to AP-1 and electrophile response elements [43,44]. Analysis of DNA regulatory regions revealed the presence of such consensus motifs in the Drosophila Gclc and Gclm promoters (S. Radyuk, unpublished). In mammals, Gclc is induced via Keap1/Nrf2 signaling; thus we examined the transcriptional profiles of cncC, (a Drosophila homologue of mammalian Nrf2 gene), and Keap1. We did not detect a circadian rhythm for either cncC or Keap1 mRNAs, nor was there any effect of per or cyc mutations on their mRNA expression levels. However, it remains possible that post-transcriptional modification of thesefactors could be involved in the temporal modulation of Gclc and Gclm expression. In contrast to the robust rhythmic expression of Gclc and Gclm mRNAs, the protein levels of GCLc did not appear rhythmic, while var.