Model of regulation of cellulase expression by ClrB, CreA, AmyR, XlnR and ClrB-2 in <i>P</i>. <i>oxalicum</i>.

<p>(A) Under cellulose growth conditions, cellulolytic inducers (cellodextrins) were liberated from cellulose by cellulase. Lack of the major intracellular beta-glucosidase Bgl2 facilitates the accumulation of intracellular cellodextrins, which can trigger signaling cascades that include expression of cellulase genes repressed by CreA and AmyR and activated by ClrB, XlnR and ClrB-2. Cellulase gene expression was synergistically enhanced by simultaneous overexpression of ClrB and lack of CreA, and additively increased in Δ<i>amyR-gpdA</i>(p)::<i>clrB</i>, Δ<i>amyR</i>-Δ<i>creA</i> and <i>gpdA</i>(p)::<i>clrB</i>-<i>gpdA</i>(p)::<i>xlnR</i> mutants. However, transcriptional regulation for cellulolytic regulator genes is also an important and powerful part of the regulatory network of cellulase gene expression. In the early cellulolytic induction, ClrB functions to repress expression of <i>amyR</i>, whose expression level is downregulated in Δ<i>creA</i>-<i>gpdA</i>(p)::<i>clrB</i> mutant and further in Δ<i>bgl2</i>-Δ<i>creA</i>-<i>gpdA</i>(p)::<i>clrB</i> mutant, and thus is activated by CreA (brown arrow) and reduced in Bgl2-deficient background. AmyR may also play a role in repressing <i>clrB</i>, <i>xlnR</i> and <i>clrB-2</i> expression on cellulose (dotted line). In addition, the novel cellulolytic regulator gene <i>clrB-2</i> expression is induced by ClrB and may be enhanced by XlnR. Moreover, transcriptional expression for ClrB, AmyR, XlnR and ClrB-2 genes is also repressed by CCR mediated by CreA in the presence of glucose (gray plunger). For each regulatory step, arrow means positive control, and plunger means negative control. (B) A “seesaw model” understanding the regulation of the cellulolytic system in transcription levels for the cellulase genes in the <i>P</i>. <i>oxalicum</i> wild-type strain. “R” means repressor, and “A” means activator. “←” means promoters for cellulase genes. (C) A “seesaw model” shows the transcription levels for cellulase genes increase by enhancing the positive regulation of activators or/and reducing the negative control of repressors (i.e. Δ<i>creA</i>, Δ<i>amyR</i>, <i>gpdA</i>(p)::<i>clrB</i>, <i>gpdA</i>(p)::<i>clrB</i>-<i>PDE_02864</i>(p)::<i>clrB</i>, <i>gpdA</i>(p)::<i>xlnR</i>, <i>gpdA</i>(p)::<i>xlnR</i>-<i>gpdA</i>(p)::<i>clrB</i>, Δ<i>creA</i>-<i>gpdA</i>(p)::<i>clrB</i>, RE-10, RE-27 and RE-29). A “seesaw model” in (D) shows the transcription levels for cellulase genes reduce by decreasing the positive regulation of activators or/and increasing the negative regulation of repressors (i.e. Δ<i>clrB</i>, Δ<i>xlnR</i>, Δ<i>clrB-</i>Δ<i>xlnR</i> and <i>gpdA</i>(p)::<i>creA</i>).