22
induction of cellulases by cellulose and sophorose, indicating that the induction by
these compounds requires Xyr1 (Akel et al. 2009). Regulation of xyr1 expression
seems to be mediated only by repression through the carbon catabolite repressorCRE1 and not by induction mechanisms (Mach-Aigner et al. 2008). ACE 1 is
believed to antagonize XYR 1 function by competing for one of its binding sites in
the promoter of genes regulated by xyr1. Thus it is a negative regulator of cellulases
and its deletion results in an enhanced expression of all the major cellulase and
hemicellulase genes in sophorose- and cellulose-induced cultures (Aro et al. 2006).
ACE 2 on the other hand is a positive regulator and Ace2 deletion has been shown
to decrease the cellulase activity in cellulose-induced cultures, while there are no
differences of gene expression in sophorose-induced cultures (Aro et al. 2001).
ACE 2 can bind the promoter motif [GGC (T/A)4] in the cbh1 promoter, which is
also recognized by XYR1. It is believed that ACE2 binding to the promoter element
requires phosphorylation and dimerization (Stricker et al. 2008). HAP 2/3/5 complex binds the CCAAT box, which is a common cis acting element found in the
promoter and enhancer region of several eukaryotic genes (Zeilinger et al. 1998).
The HAP 2/3/5 complex binding is believed to generate an open chromatin structure
for complete transcriptional activation. Sophorose induction can cause the loss of
nucleosome positioning in the promoter mediated by binding of the HAP 2/3/5
complex, which in turn makes the TATA box accessible (Zeilinger et al. 2003). Read
together, the above details present a model for cellulase regulation in which the
master regulator XYR1 is fine-tuned by regulators like ACE1 and ACE2. While
XYR1 is nonsubstrate specific, ACE1 and ACE 2 are more specific, helping to finetune cellulase gene regulation. The role of the HAP 2/3/5 complex is proposed to be
facilitating the binding of other factors to the cellulase promoter by removing
nucleosome positioning on the DNA. More recently, the role of another cellulase
regulator designated as ACE III has been proposed, which is essential for the expression of several cellulase genes (Häkkinen et al. 2014). Apparently, the knowledge
about cellulase regulation has come a long way and is progressing, and this can have
serious impacts on our understanding of plant cell wall degradation as well as on
how fungal strains are developed for commercial production of biomass-degrading
enzymes. The above discussion has been focused on the regulation of cellulases in
T. reesei, and while it cannot be generalized, similar mechanisms of regulation exist
in other fungi. More detailed discussions on the regulation of cellulase gene expression in T. reesei and other fungi may be found in Amore et al. (2013).
1.5
Cellulase Production Strategies
Production of cost-effective cellulase preparations for biomass hydrolysis is the
major challenge in successful development of biofuels, despite the fact that there
have been significant improvements in production technologies that have brought
down the cost of enzymes per unit amount of ethanol produced from biomass. Also
the cost evaluations of cellulases have been complicated due to the fact that there is
scant information available in public about the technologies and raw materials used
R.K. Sukumaran et al.
induction of cellulases by cellulose and sophorose, indicating that the induction by
these compounds requires Xyr1 (Akel et al. 2009). Regulation of xyr1 expression
seems to be mediated only by repression through the carbon catabolite repressorCRE1 and not by induction mechanisms (Mach-Aigner et al. 2008). ACE 1 is
believed to antagonize XYR 1 function by competing for one of its binding sites in
the promoter of genes regulated by xyr1. Thus it is a negative regulator of cellulases
and its deletion results in an enhanced expression of all the major cellulase and
hemicellulase genes in sophorose- and cellulose-induced cultures (Aro et al. 2006).
ACE 2 on the other hand is a positive regulator and Ace2 deletion has been shown
to decrease the cellulase activity in cellulose-induced cultures, while there are no
differences of gene expression in sophorose-induced cultures (Aro et al. 2001).
ACE 2 can bind the promoter motif [GGC (T/A)4] in the cbh1 promoter, which is
also recognized by XYR1. It is believed that ACE2 binding to the promoter element
requires phosphorylation and dimerization (Stricker et al. 2008). HAP 2/3/5 complex binds the CCAAT box, which is a common cis acting element found in the
promoter and enhancer region of several eukaryotic genes (Zeilinger et al. 1998).
The HAP 2/3/5 complex binding is believed to generate an open chromatin structure
for complete transcriptional activation. Sophorose induction can cause the loss of
nucleosome positioning in the promoter mediated by binding of the HAP 2/3/5
complex, which in turn makes the TATA box accessible (Zeilinger et al. 2003). Read
together, the above details present a model for cellulase regulation in which the
master regulator XYR1 is fine-tuned by regulators like ACE1 and ACE2. While
XYR1 is nonsubstrate specific, ACE1 and ACE 2 are more specific, helping to finetune cellulase gene regulation. The role of the HAP 2/3/5 complex is proposed to be
facilitating the binding of other factors to the cellulase promoter by removing
nucleosome positioning on the DNA. More recently, the role of another cellulase
regulator designated as ACE III has been proposed, which is essential for the expression of several cellulase genes (Häkkinen et al. 2014). Apparently, the knowledge
about cellulase regulation has come a long way and is progressing, and this can have
serious impacts on our understanding of plant cell wall degradation as well as on
how fungal strains are developed for commercial production of biomass-degrading
enzymes. The above discussion has been focused on the regulation of cellulases in
T. reesei, and while it cannot be generalized, similar mechanisms of regulation exist
in other fungi. More detailed discussions on the regulation of cellulase gene expression in T. reesei and other fungi may be found in Amore et al. (2013).
1.5
Cellulase Production Strategies
Production of cost-effective cellulase preparations for biomass hydrolysis is the
major challenge in successful development of biofuels, despite the fact that there
have been significant improvements in production technologies that have brought
down the cost of enzymes per unit amount of ethanol produced from biomass. Also
the cost evaluations of cellulases have been complicated due to the fact that there is
scant information available in public about the technologies and raw materials used
R.K. Sukumaran et al.