The Neurospora crassa OS MAPK pathway-activated transcription factor ASL-1 contributes to circadian rhythms in pathway responsive clock-controlled genes

Jan 14, 2012Fungal genetics and biology : FG & B

The ASL-1 protein activated by the OS MAPK pathway helps control daily rhythms in specific clock-regulated genes in Neurospora crassa

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Abstract

Three osmotic stress-induced genes, cat-1, gcy-1, and gcy-3, exhibit circadian rhythms in expression.

The OS-pathway MAPK cascade in Neurospora crassa is linked to the response to osmotic stress.
Activation of the OS-2 MAPK induces the transcription of multiple genes related to osmotic stress.
Newly identified rhythmic genes encode a catalase and two glycerol dehydrogenases, suggesting their role in glycerol production.
Circadian rhythms in these genes depend on the oscillator component FRQ.
ASL-1, an OS-responsive transcription factor, is crucial for maintaining circadian rhythms of certain OS-pathway responsive transcripts.
ASL-1 mRNA levels do not show rhythmicity, indicating that post-transcriptional regulation by OS-2 may influence downstream gene rhythms.

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The OS-pathway mitogen-activated protein kinase (MAPK) cascade of Neurospora crassa is responsible for adaptation to osmotic stress. Activation of the MAPK, OS-2, leads to the transcriptional induction of many genes involved in the osmotic stress response. We previously demonstrated that there is a circadian rhythm in the phosphorylation of OS-2 under constant non-stress inducing conditions. Additionally, several osmotic stress-induced genes are known to be regulated by the circadian clock. Therefore, we investigated if rhythms in activation of OS-2 lead to circadian rhythms in other known stress responsive targets. Here we identify three more osmotic stress induced genes as rhythmic: cat-1, gcy-1, and gcy-3. These genes encode a catalase and two predicted glycerol dehydrogenases thought to be involved in the production of glycerol. Rhythms in these genes depend upon the oscillator component FRQ. To investigate how the circadian signal is propagated to these stress induced genes, we examined the role of the OS-responsive transcription factor, ASL-1, in mediating circadian gene expression. We find that while the asl-1 transcript is induced by several stresses including an osmotic shock, asl-1 mRNA accumulation is not rhythmic. However, we show that ASL-1 is required for generating normal circadian rhythms of some OS-pathway responsive transcripts (bli-3, ccg-1, cat-1, gcy-1 and gcy-3) in the absence of an osmotic stress. These data are consistent with the possibility that post-transcriptional regulation of ASL-1 by the rhythmically activated OS-2 MAPK could play a role in generating rhythms in downstream targets.

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The Neurospora crassa OS MAPK pathway-activated transcription factor ASL-1 contributes to circadian rhythms in pathway responsive clock-controlled genes

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