Catabolite repression

One of the advantages of the fed-batch fermentation is the fact that the residual substrate concentration may be maintained at a very low level. This may result in a removal of catabolite repressive effects and avoidance of toxic effects of medium components. 

Fed-batch mixed reactor starting with a relatively dilute solution of substrate this provides control over the substrate concentration. High rates are avoided. Fed batch is used for baker s yeast to overcome catabolite repression and to control oxygen demand. It is also used routinely for production of Penicillin. 

Erwinia chrysanthemi synthesizes and secretes a large number of pectinases. The major pectinases include a pectin methylesterase PemA and five isoenzymes of endo-pectate lyases PelA, PelB, PelC, PelD and PelE. In addition, secondary pectinases were identified a pectin methylesterase PemB, two endo-pectate lyases PelL and PelZ, an exo-pectate lyase PelX and an exopolygalacturonase, PehX. The regulation of pectinase synthesis is very complex and dependent on many environmental conditions. It is induced by pectin catabolic products and affected by growth phase, catabolite repression, osmolarity, iron or oxygen starvation... 

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 screening method based on diameter of halos yielded only five strains with at least 20% more PG than wild t> e. Most attempts to enhance pectinase production have been made in filamentous fiingi, in which enzyme production is regulated by induction and catabolite repression mechanisms [8], By contrast, PG produced by marxianus is constitutive and not subject to catabolite repression [6], Failure to find greatly enhanced secretion levels is consistent with the idea of a constitutive gene that is capable of only modest further induction. PG was found in this study to be already the most prolific secreted protein of K. marxianus. 

Nitrogen-catabolite repression (NCR) and nitrogen-catabolite inactivation (NCI) two superimposed regulatory mechanisms affecting uptake systems for nitrogenous compounds... 

Nitrogen-catabolite repression of GAPl synthesis has been studied in mutant strains containing mutations such as npil or npi2 which destroy the permease inactivation mechanism. With the help of these mutants, it could be clearly demonstrated that GAPl synthesis is repressed in cells grown in the presence of ammonium ions. 

Duetz WA, S Marques, C de Jong, JL Ramos, J G van Andel (1994) Inducibility of the TOL catabolic pathway in Pseudomonas putida (pWWO) growing on succinate in continuous culture evidence of carbon catabolite repression control. J Bacteriol 176 2354-2361. 

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. 

Avoiding Substrate Inhibition, Enzymatic Inhibition, and Catabolite Repression When using the fed-batch operation, the concentration of a substrate can be kept at a low value so that substrate inhibition, enzymatic inhibition, or catabolite repression is practically avoided, which results in high productivity [14]. 

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). 

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