Diauxic growth curve effect of E.coli

Many enzymes are involved in basic cellular housekeeping functions and are synthesized at a more or less constant rate. These are called constitutive enzymes. Other enzymes are synthesized at rates that vary with the cells circumstances and are termed as adaptative or inducible enzymes.

Lactose metabolizing enzymes are inducible:

Bacteria adapt to their environment by producing enzymes that metabolize certain nutrients e.g., lactose, only when these substances are available. E.coli growing in the absence of lactose are initially unable to metabolize this disaccharide to do so, they require the presence of two-protein- beta galactosidase which catalyses the hydrolysis of lactose to its component- monosaccharide and galactosidase permease or lactose permease which transport lactose into the cell. Lactose or one of its metabolic products most somehow trigger the synthesis of above proteins. Such a substance is known as an inducer. The physiological inducer of the lactose system is 1,6 allolactose but isopropyl thiogalactosidase (IPTG) is also a potent inducer which structurally resemble allolactose-lactose system inducers also stimulate the synthesis of thiogalactosidase-trans-acetylase, an enzyme that transfer an acetyl group from acetyl CoA to the 6-OH group of isopropyl-thiogalactosidase such as IPTG.

Lac system genes form an operon- LAC OPERON

The genes specifying wild type beta-galactosidase, lactose permease and thiogalactosidase transacetylase are designated as z, y and a respectively. These are called lac structural genes (genes that specify polypeptide) and are contiguously arranged on E.coli chromosome.

These genes together with the control element 'p and o' form a genetic unit called an operon. Specifically the lac operon. In the absence of inducer, the lac repressor i.e., the regulatory gene prgene product specifically binds tightly to the O gene so as to prevent the transcription of mRNA. On binding inducer, the repressor dissociates from the operator thereby permitting the transcription and subsequent translation of lac enzyme.

Catabolite repression

Glucose is E.coli metabolite of choice the availability of adequate amount of glucose prevents the full expression of genes that encodes protein involved in the fermentation of numerous other catabolites, including lactose, arabinose, galactose, even when these metabolites are present in high concentration. This phenomenon is known as "catabolite repression" which prevents the wasteful duplication of energy producing energy system.

If E.coli grows in a medium containing both glucose and lactose, it uses glucose preferentially until the sugar is exhausted. Then after a short lag, growth resumes with lactose as the carbon source, this biphasic growth pattern or response is called 'diauxic growth'.

cAMP signals the lack of glucose

The greatly diminished level of cAMP in the presence of glucose is the indication that of mechanism of catabolite repression. The increase in cAMP may be due to the effect of the phosphoenolpyruvate phospho-transferase system (PTS) on the adenyl activity, the enzyme that synthesizes cAMP. Enzyme III of PTS denotes a phosphate to glucose during its transport, therefore it enters the cell as glucose-6 phosphate. The phosphorylated form of enzyme III also activates adenyl cyclase.

If glucose is being rapidly transported by PTS the amount of phosphorylated enzyme III is low and the adenyl cyclase is less active, so the cAMP level drops. At least one other mechanism is involved in diauxic growth. When the PTS is actively transported glucose into the cell, non-phosphorylated enz III is more prevalent non-phosphorylated enz III binds to the lactose permease and allosterically inhibits it, thus blocking lactose uptake.

CAP-cAMP complex stimulates transcription of catabolite repressed operons:

CAP is homodimer of 210 residue subunits that undergo a large conformational change on binding cAMP. This CAP is synonymously called as catabolite gene activator protein or cAMP Receptor Protein (CRP).

CAP-(cAMP)2 complex but not CAP itself binds to lac-operon and stimulates transcription from its otherwise low efficiency promoter in the absence of lac repressor. CAP is therefore a positive regulator in contrast to the lac repressor which is a negative regulator (twins off transcription).