Conserved and essential transcription factors for cellulase gene expression in ascomycete fungi

Abstract

Rational engineering of filamentous fungi for improved cellulase production is hampered by our incomplete knowledge of transcriptional regulatory networks. We therefore used the model filamentous fungus Neurospora crassa to search for uncharacterized transcription factors associated with cellulose deconstruction. A screen of a N. crassa transcription factor deletion collection identified two uncharacterized zinc binuclear cluster transcription factors (clr-1 and clr-2) that were required for growth and enzymatic activity on cellulose, but were not required for growth or hemicellulase activity on xylan. Transcriptional profiling with nextgeneration sequencing methods refined our understanding of the N. crassa transcriptional response to cellulose and demonstrated that clr-1 and clr-2 were required for the bulk of that response, including induction of all major cellulase and some major hemicellulase genes. Functional CLR-1 was necessary for expression of clr-2 and efficient cellobiose utilization. Phylogenetic analyses showed that CLR-1 and CLR-2 are conserved in the genomes of most filamentous ascomycete fungi capable of degrading cellulose. In Aspergillus nidulans, a strain carrying a deletion of the clr-2 homolog (clrB) failed to induce cellulase gene expression and lacked cellulolytic activity on Avicel. Further manipulation of this control system in industrial production strains may significantly improve yields of cellulases for cellulosic biofuel production.

ManR, a Transcriptional Regulator of the β-Mannan Utilization System, Controls the Cellulose Utilization System in <i>Aspergillus oryzae</i>

“…19) In the present study, we found that ManR regulates not only mannanolytic enzyme genes but also cellulolytic enzyme genes in A. oryzae. Hence the regulatory function of cellulase gene expression is conserved among these orthologous genes, manR, clrB and clr2.…”

“…Hence the regulatory function of cellulase gene expression is conserved among these orthologous genes, manR, clrB and clr2. However, although deletion of clrB caused a severe reduction in the production of xylanolytic enzymes in A. nidulans when the ÁclrB mutant was grown in a cultivation medium containing Avicel, 19) disruption of manR did not significantly affect the expression of xylanolytic enzyme genes in A. oryzae when Avicel was used as sole carbon source (data not shown). Hence the system regulating hemicellulase genes in A. oryzae might be different from that in A. nidulans.…”

Transcriptional Regulation and Responses in Filamentous Fungi Exposed to Lignocellulose

“…Alleviation of CCR, either by inactivation of the CreA/CRE1 regulator or by lack of a carbon source in the medium, results in the increase of transcription of genes encoding CAZymes that are active on plant-derived carbohydrates in a number of fungi [42][43][44][45][46][47][48]. A large proportion of these genes was expressed both during carbon starvation conditions and during exposure to lignocellulose for example in N. crassa exposed to Avicel [45] or A. niger exposed to wheat straw [42]. This transcriptional response to carbon starvation results in secretion of enzymes that are active on plant-derived carbohydrates [42].…”

“…That study defined the 'pectin regulon' and compared this to the 'Avicel regulon' from Coradetti et al [45] and the 'xylan regulon' -the xylan study was a replication of the microarray experiment from Sun et al [56] using RNAseq. All three substrates significantly induced a common set of 29 genes and three quarters of these genes encoded CAZymes.…”

Effect of different terminators on transcription regulatory factor ClrB and XlnR in Penicillium oxalicum 114-2

“…It is reported that the degradation intensity of cellulase is depends on the transcription regulatory factor. ClrB is a global positive regulator,and ClrB/CLR -2, is essential for cellulase expression in P. oxalicum, N.crassa and Aspergillus nidulans [7,8] . It also belongs to the zinc finger protein transcription factor superfamily.…”