21
known as carbon catabolite repression (Aro et al. 2005). Understanding the regulation of cellulase gene expression in fungi is important for designing strategies as
well as for genetic manipulations to enhance cellulase production.
Production of most of the cellulases/hemicellulases is coordinately regulated and
induced in the presence of natural cellulosic substrates. How the insoluble (hemi)
cellulose polymers can induce gene expression was a question that intrigued
researchers, and it was speculated from quite early the role of a diffusible small
molecule inducer, originally identified as sophorose (2-O-β glucopyranosyl glucose), a disaccharide believed to be generated by transglycosylation activity of
β-glucosidases (Sternberg and Mandels 1979, 1980). Other pure sugars/oligosaccharides like xylose, xylobiose, gentiobiose, sorbose, galactose, lactose, β-cellobiono
1, 5 lactone, etc. have been reported as inducers of (hemi)cellulases (Aro et al. 2005;
Amore et al. 2013). The proposed mechanism of cellulase induction assumes that
there is a basal-level expression of cellulases, especially Cel7A and Cel6A (cellobiohydrolase I and II), and the extracellular cellulase activity results in the generation of the soluble inducer. Evidence of this came from the work of Carle-Urioste
et al. (1997), who demonstrated that the mRNAs of cbh1 and egl1 are transcribed
under uninduced conditions and, on induction, the transcript numbers can increase
to more than 1000-fold compared to the basal expression. It is proposed that the
rate-limiting step in cellulase induction is the initial hydrolysis of cellulose. About
14 h is required for induction and this is thought to be because of the dependency on
protein synthesis (Amore et al. 2013). Cel6A (CBH2) is the main surface-bound
cellulase in T. reesei and is implicated in the synthesis of soluble inducer (Seiboth
et al. 1992). Sophorose, the best-known inducer for cellulases, is synthesized from
cellobiose through transglycosylation activity of β glucosidase (Vaheri et al. 1979).
While the involvement of the major extracellular BGL – Cel3A – is proven (Fowler
and Brown 1992), it is not the sole BGL that is implicated in the synthesis of sophorose (Amore et al. 2013).
Though the complete pathway of cellulase induction by sophorose is not known,
sufficient information exists on the transcription factors that are involved in the
regulation of cellulases and hemicellulases. Five transcription factors have been
identified to be involved in the regulation of cellulases and these include the positive
regulators XYR1, ACE 2, and HAP2/3/5 complex and the negative regulators ACE
1 and CRE1 (Kubicek et al. 2009). There are also recent reports on other factors that
regulate the expression of cell wall-degrading enzymes in T. reesei which include
the transcription factor BglR (Nitta et al. 2012) and the putative methyl transferase
LAE1 (Seiboth et al. 2012b). The former is suggested to regulate the BGL genes
whereas the latter is considered important for production of cellulases and hemicellulases, while the precise mechanism is still unclear (Häkkinen et al. 2014). Xyr1
acts as the central regulator of plant cell wall degradation in T. reesei, and controls
the expression of CAZYme genes such as xyn1, xyn2, bxl1, cbh1, cbh2, egl1, and
bgl1(Stricker et al. 2006). While XYR1 mediates the induction signal from various
inducing carbon sources for these genes, its transcription is not induced by any of
these inducers (Mach-Aigner et al. 2008). Deletion of xyr1 results in lack of
1 Enzymes for Bioenergy
known as carbon catabolite repression (Aro et al. 2005). Understanding the regulation of cellulase gene expression in fungi is important for designing strategies as
well as for genetic manipulations to enhance cellulase production.
Production of most of the cellulases/hemicellulases is coordinately regulated and
induced in the presence of natural cellulosic substrates. How the insoluble (hemi)
cellulose polymers can induce gene expression was a question that intrigued
researchers, and it was speculated from quite early the role of a diffusible small
molecule inducer, originally identified as sophorose (2-O-β glucopyranosyl glucose), a disaccharide believed to be generated by transglycosylation activity of
β-glucosidases (Sternberg and Mandels 1979, 1980). Other pure sugars/oligosaccharides like xylose, xylobiose, gentiobiose, sorbose, galactose, lactose, β-cellobiono
1, 5 lactone, etc. have been reported as inducers of (hemi)cellulases (Aro et al. 2005;
Amore et al. 2013). The proposed mechanism of cellulase induction assumes that
there is a basal-level expression of cellulases, especially Cel7A and Cel6A (cellobiohydrolase I and II), and the extracellular cellulase activity results in the generation of the soluble inducer. Evidence of this came from the work of Carle-Urioste
et al. (1997), who demonstrated that the mRNAs of cbh1 and egl1 are transcribed
under uninduced conditions and, on induction, the transcript numbers can increase
to more than 1000-fold compared to the basal expression. It is proposed that the
rate-limiting step in cellulase induction is the initial hydrolysis of cellulose. About
14 h is required for induction and this is thought to be because of the dependency on
protein synthesis (Amore et al. 2013). Cel6A (CBH2) is the main surface-bound
cellulase in T. reesei and is implicated in the synthesis of soluble inducer (Seiboth
et al. 1992). Sophorose, the best-known inducer for cellulases, is synthesized from
cellobiose through transglycosylation activity of β glucosidase (Vaheri et al. 1979).
While the involvement of the major extracellular BGL – Cel3A – is proven (Fowler
and Brown 1992), it is not the sole BGL that is implicated in the synthesis of sophorose (Amore et al. 2013).
Though the complete pathway of cellulase induction by sophorose is not known,
sufficient information exists on the transcription factors that are involved in the
regulation of cellulases and hemicellulases. Five transcription factors have been
identified to be involved in the regulation of cellulases and these include the positive
regulators XYR1, ACE 2, and HAP2/3/5 complex and the negative regulators ACE
1 and CRE1 (Kubicek et al. 2009). There are also recent reports on other factors that
regulate the expression of cell wall-degrading enzymes in T. reesei which include
the transcription factor BglR (Nitta et al. 2012) and the putative methyl transferase
LAE1 (Seiboth et al. 2012b). The former is suggested to regulate the BGL genes
whereas the latter is considered important for production of cellulases and hemicellulases, while the precise mechanism is still unclear (Häkkinen et al. 2014). Xyr1
acts as the central regulator of plant cell wall degradation in T. reesei, and controls
the expression of CAZYme genes such as xyn1, xyn2, bxl1, cbh1, cbh2, egl1, and
bgl1(Stricker et al. 2006). While XYR1 mediates the induction signal from various
inducing carbon sources for these genes, its transcription is not induced by any of
these inducers (Mach-Aigner et al. 2008). Deletion of xyr1 results in lack of
1 Enzymes for Bioenergy