Biochemical Engineering Fundamentals Part 1 - Bailey J., Ollis D. 1989

Molecular genetics and regulatory systems
Industrial applications of research results in microbial genetics and mutant populations
Cellular regulation systems; influence of medium composition

The achievements of molecular biology and insights into the regularities of cellular regulation systems have significant Structure/179.html">Practical Applications. Some of these applications, related to identifying the Location of a desired or harmful Gene and subsequently modifying it through various techniques, were discussed in the previous section. Another approach is based on inducing Mutations to obtain a more productive strain; we already mentioned this in Chapter 1 in connection with increasing penicillin yields through The Use of special strains derived via a series of sequential mutations. In the final part of this section, we will examine several other microbiological processes that rely primarily on specially bred mutant strains.

Often, the enhanced productivity of a mutant microorganism is readily explained by alterations in The Cell's core regulatory systems. Understanding the mechanisms that control The Biosynthesis of a desired metabolic product is also crucial because only on this basis can informed decisions be made regarding the nutrient medium composition and control Methods required for maximum productivity. We will examine the Influence of the medium on cell productivity first.

There are two approaches to solving The problem of increasing cell productivity by modifying the medium composition: first, inducing substances can be introduced into the medium, and second, an attempt can be made to lower the concentration of the repressor. At first glance, both approaches seem quite straightforward, but their practical Implementation can entail A number of difficulties.

Beyond the substrate itself, non-metabolizable substrate analogues can serve as exceptionally potent Inducers of enzyme biosynthesis. For instance, galactosides—specifically isopropyl-β-D-thiogalactoside—can increase the specific β-galactosidase activity of E. coli by more than 1,000-fold. Analogues of other substrates are effective for their respective Enzymes; thus, N-acetylacetamide enhances amidase activity, methicillin accelerates the biosynthesis of penicillin-β-lactamase, and malonic acid promotes maleate isomerase activity.

It is well known that in many practically important processes, the biosynthesis of a metabolic product can be inhibited by catabolite repression. The availability of a rapidly assimilated substrate, such as relatively concentrated glucose solutions, stimulates cell growth while simultaneously suppressing the synthesis of Antibiotics like penicillin, mitomycin, bacitracin, and streptomycin. Penicillin yields are significantly enhanced under conditions of artificial diauxie (Fig. 7.14), i.e., by adding a slowly metabolized sugar, such as lactose, to the glucose-containing nutrient medium. Under such conditions, biomass grows on glucose, whereas penicillin synthesis proceeds as the lactose is assimilated. The exact same effect can be achieved alternatively by feeding glucose slowly into the nutrient medium.

Other Examples of repressors whose concentrations are easily regulated by altering the medium composition include inorganic phosphate—which suppresses the synthesis of phosphatase in E. coli and nuclease in Aspergillus quernicus—and ammonia, which inhibits urease biosynthesis in Proteus rettgeri. For example, by lowering the phosphate ion concentration in the growth medium, the alkaline phosphatase content in E. coli can be increased from an almost undetectable level to approximately 5% of the total cellular protein mass. Similarly, decreasing the concentration of Amino Acids or sulfate ions significantly accelerates protease synthesis by Bacteria or Algae such as Aspergillus niger, respectively.

It is more challenging to maintain a low concentration of a repressor if it is synthesized by the microorganism itself. One potential strategy to address this challenge is to modify The cell membrane so that the repressor readily diffuses out of the cell and into the medium. This approach is employed, for instance, in the highly efficient production of glutamic acid using Corynebacterium glutamicum, where the release of glutamic acid into the medium is triggered by a reduction in biotin concentration. In the presence of excess biotin, the intracellular concentrations of Other Amino Acids reach a maximum. Although the exact mechanism of this effect remains unknown, certain experimental evidence suggests that biotin influences the phospholipid Components of the cell membrane.

In the following section, we will discuss other methods for mitigating the negative effects of repressors.