Involvement of a helix–loop–helix transcription factor CHC-1 in CO2-mediated conidiation suppression in Neurospora crassa

Oct 18, 2011Fungal genetics and biology : FG & B

Role of the CHC-1 protein in how CO₂ reduces spore formation in Neurospora crassa

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Abstract

A mutant of Neurospora crassa lacking the gene NCU00749 produced abundant conidia in conditions that suppressed conidiation in the wild-type strain.

The gene NCU00749 encodes a protein with a helix-loop-helix DNA binding region.
Enhanced conidiation in the NCU00749 deletion mutant was only slightly affected by the antioxidant N-acetyl cysteine.
The NCU00749 mutant formed conidia earlier than the wild-type strain, especially at higher CO(2) concentrations.
In the absence of NCU00749, genes associated with conidial development were transcribed at higher rates compared to the wild-type strain.
RNA sequencing revealed that 404 genes had significant changes in transcription due to the deletion of NCU00749.
The deletion of NCU00749 influenced genes involved in carbon metabolism, cell wall synthesis, and calcium signaling, beyond its role in conidiation.

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The morphological switch from vegetative growth to conidiation in filamentous fungi is highly regulated, but the understanding of the regulatory mechanisms is limited. In this study, by screening a set of knock-out mutants corresponding to 103 transcription factor encoding genes in Neurospora crassa, a mutant was found to produce abundant conidia in race tubes in which conidiation in the wild-type strain was suppressed. The corresponding gene NCU00749 encodes a protein containing a helix-loop-helix DNA binding region. Unlike enhanced conidiation in ras-1 and sod-1 mutants, which was completely suppressed by antioxidant N-acetyl cysteine, enhanced conidiation in the NCU00749 mutant was only slightly affected by N-acetyl cysteine. When grown on slants, the NCU00749 deletion mutant exhibited earlier conidial formation than the wild-type strain, and this was more evident at a higher (5%) CO(2) concentration. Therefore, we named NCU00749 as conidiation at high carbon dioxide-1 (chc-1). Genes that are highly expressed during conidial development, eas, con-6, con-8 and con-10, were transcribed at a higher rate in the chc-1 deletion mutant than the wild-type strain in response to conidiation induction. To determine the mechanisms by which CHC-1 regulates conidiation, we conducted a RNA sequencing analysis and found that 404 genes exhibited ≥ 2 fold changes in transcription in response to chc-1 deletion. Among them, fluffy and ada-6, two transcription factor genes that positively regulate conidiation in N. crassa, and rca-1, whose homolog flbD in Aspergillus nidulans is essential for conidiation, were upregulated in the chc-1 deletion mutant. Results of RNA sequencing also suggest that signal transduction via the cAMP and the MAK-2 mediated signal pathways, and ROS generation and removal, mechanisms known to regulate conidiation, are not involved in chc-1 mediated control of conidiation. In addition, chc-1 also influences expression of genes involved in other important biological processes besides conidiation such as carbon metabolism, sphingolipid synthesis, cell wall synthesis, and calcium signaling.

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Involvement of a helix–loop–helix transcription factor CHC-1 in CO2-mediated conidiation suppression in Neurospora crassa

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