Catabolite repression

Catabolite repression

Cellulase synthesis in Trichoderma is regulated by catabolite repression as well as by induction.When sophorose was added to the glycerol medium after 48 h culture of EA3-867，the cellulase was produced after 12 h，reached its maximum after 24 h，and then decreased.However，the cellulase increased again by new supplementation of sophorose on the fourth day of culture，an observation explained by the depletion of sophorose in the medium via metabolism，as shown the sophorose determination.Glycerol，however，distinctly repressed the cellulase production induced by sophorose.［5］More Convincing evidence for the regulation of cellulase synthesis by catabobte repression was the finding that cellulase synthesis of washed mycelium was inhibited to a different degree by various，easily utilizable carbon sources，such as glucose，fructose，galactose，ribose，glucose-6-phosphate，6-phosphogluconate，gluconate，glycerol，as well as organic acids（10-3—10-2 M）of the tricarboxylic acid cycle（acetate，pyruvate，citrate，x-ketoglutarate，malate，succinate，fumarate or oxaloacetate）［6］The cellulase induction usually recovered from repression after 24 h of culture.It is of interest that the catabolite repression of glucose could not be overcome by cyclic AMP（see Table 2）.

Cellulase induction by glucose molasses and extracts of pods of Sophora japonica L.

Since cellulase synthesis is regulated both by induction and catabolite repression，the principle of increasing cellulase production should be based on the induction and derepression at the optimal conditions for rapid growth of mycelium as well as cellulase synthesis.

Although sophorose can strongly induce cellulase production，the commercial application of this rare sugar is not practical.Sophorose is a byproduct in glucose production.It was thus likely that the glucose molasses should contain more sophorose as an impurity.［7］The glucose molasses used for cellulase induction in our experiments was the waste liquor after the first crystallization of glucose in glucose production from corn starch by hydrochloric acid hydrolysis（in a Shanghai glucose refinery）.It has been shown that glucose molasses has strong inductive activity for cellulase production in washed mycelium，probably in a similar manner to sophorose plus glucose.Cellulase production on glucose molasses does not commence until glucose is consumed；the remaining sophorose then exerts its inductive effect（see Table 3）.However，no cellulase was induced by glucose molasses at higher concentration（1%）as a result of the catabolite repression by glucose itself.

The molasses were separated by charcoal and paper chromatography into four components：glucose（RG 1.00），sophorose（RG 0.42），gentiobiose（RG 0.26），and an unknown triose（RG 0.08）.Only sophorose was identified to be responsible for cellulase induction.Gentiobiose only had feeble inductive activity.［7］The extracts of pods of Sophora japonica L，after hydrolysis with diluted H2 SO4，had similar ability to induce the ceUulase production（see Figure 7）.It was shown that the extract contained sophorose，glucose，fructose，sucrose and other oligosaccharides.

In order to increase cellulase production and shorten culture time，it is suggested that in the early period of culture，or log phase of Trichoderma（first 48 h），optimal conditions for mycelial growth should be provided to promote rapid use of sugar.After 48 h in culture，Trichoderma grows to the stationary phase.Cellulase may then be induced by a strong inducer（such as sophorose）without catabolite repression at the optimal condition for cellulase synthesis.In fact，glucose molasses is an ideal inducer.When glucose molasses was added to a simple medium，the mutant N2-78 reached a quite high activity level in only 60 h of culture.In contrast，Trichoderma may require one to two weeks to reach the maximum of cellulase production.［18，19］