Glucose catabolite repression

PG production was constitutive and not subject to carbon catabolite repression. Highest yields were on glucose and fructose (upto 10% w/v) (Table 1). Inclusion of pectic compounds had no effect on growth or PG production (data not shown). 

The work to be summarized here is preliminary and will be published elsewhere (manuscripts in preparation). Thermoanaerobacter strain B6A produces a xylanase complex diat is inducible by xylose and catabolite repressed by glucose. Noticeably, when mixed with insoluble xylan, glucose-grown cells do not bind xylan whereas, xylose-grown cells bind very tightly. Canonized ferritin was used to identify xylanosome structures in xylose grown cells which are similar to cellulosomes observed in C. thermocellum, Thermoanaerobacter strain B6A produces thermostable cell-bound endoxylanase, which is also excreted. 

Glucose is the preferred carbon source for yeast, as it is for bacteria. When glucose is present, most of the GAL genes are repressed—whether galactose is present or not. The GAL regulatory system described above is effectively overridden by a complex catabolite repression system that includes several proteins (not depicted in Fig. 28-29). 

Catabolite repression. The general repression of transcription of genes associated with catabolism that is seen in the presence of glucose. 

In addition to catabolite repression, the cellulase enzymes themselves are subject to end-product inhibition. For example, as glucose accumulates during saccharification, it interacts noncompetively with cellobiase to inhibit further activity of this enzyme (6). Similar inhibition of endoglucanases occurs when cellobiose accumulates in a saccharification reactor (18,19,20). 

Catabolite repression is a two-part system. The first component is the small-molecule regulator, cyclic AMP. Glucose decreases cyclic AMP synthesis. The second component is cyclic AMP binding protein, CAP. CAP binds cAMP and thereby helps RNA polymerase bind to the promoter. When bound to cAMP, CAP binds to a sequence at the 5 end of the lac promoter. CAP binding bends the DNA, allowing protein-protein contact between CAP and polymerase. It therefore behaves in the opposite manner of repressor. Repressor (LacI) binds to operator DNA only in the absence of its small-molecule ligand, while CAP binds to promoter DNA in the presence of its small-molecule ligand. 

Professor Fiechter succeeded to show using the improved chemostat technique that glucose and oxygen influence various yeast stains differently. Beside the catabolite repression (glucose effect) a second regulation type exists which is controlled by the dynamic substrate flux (glucose). This causes different types... 

Although the inducer causes the lacl repressor protein to dissociate from the operator, the glucose will lead to a low level of cAMP (low adenylate cyclase activity). As the CAP-cAMP complex dissociates, the CAP protein will leave the CAP site. This will repress transcription of lacZ, lac Y, and lac A (catabolite repression). 

Second, carbon and nitrogen sources, pH, temperature, and water activity are important nutritional and environmental factors for AF production.199 It is known that AF production is induced by glucose or sucrose, but not induced by peptone or lactose, suggesting that a catabolite repression is involved in AF production. AF production is strongly suppressed by the presence of nitrate or under an alkaline condition. AF contamination in crops is observed only in tropical and semitropical areas with optimal environmental conditions for AF production by fungi. 

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