Diauxie

Diauxie is a French word meaning double growth. The word is used in English in cell biology to describe the growth phases of a bacterial colony as it metabolizes a mixture of sugars. During the first phase, cells preferentially metabolize the sugar whose catabolism is most efficient (often glucose). Only after the first sugar has been exhausted do the cells switch to the second. At the time of the "diauxic shift", there is often a lag period during which the cell produces the enzymes needed to metabolize the second sugar.

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Jacques Monod discovered diauxic growth in 1941 during his experiments with Escherichia coli and Bacillus subtilis. His work led to the operon model of gene expression.

Diauxie occurs because organisms use multiple sets of genes to metabolize the different nutrients they encounter. If an organism allocates energy to metabolize nutrients that are processed inefficiently or that are not highly abundant, it may be put at a reproductive disadvantage. Through evolution, organisms have developed the ability to change their genetic activity so as only to express those genes resulting in the most efficient energy production. For example, when grown in the presence of both glucose and maltose, Lactococcus lactis will produce enzymes to metabolize glucose first, altering its gene expression to use maltose only after the supply of glucose has been exhausted. In the case of yeast (a fungus), the diauxic shift frequently represents a change in metabolism from glucose fermentation to aerobic respiration as the glucose is depleted. This shift is accompanied by a change from the usage of glycolysis to gluconeogenesis and the glyoxylate cycle.^[1]

Mechanism

In the 1940s, Monod hypothesized that a single enzyme could adapt to metabolize different sugars. It took 15 years of further work to show that this was incorrect. During his work on the lac operon of E. coli, Joshua Lederberg isolated β-galactosidase and found it in greater quantities in colonies grown on lactose compared to other sugars. Melvin Cohn in Monod's lab at the Pasteur Institute then found that β-galactosides induced enzyme activity. The idea of enzyme adaptation was thus replaced with the concept of enzyme induction, in which a molecule induces expression of a gene or operon, often by binding to a repressor protein and preventing it from attaching to the operator.^[2]

In the case of the bacterial diauxic shift from glucose to lactose metabolism, glucose initially inhibits the ability of the enzyme adenylate cyclase to synthesize cyclic AMP (cAMP). cAMP, in turn, is required for the catabolite activator protein (CAP) to bind to DNA and activate the transcription of the lac operon, which includes genes necessary for lactose metabolism. The presence of lactose is sensed through the activity of the lac repressor, which inhibits transcription of the lac operon unless it is bound by lactose. Thus, if glucose is present, cAMP levels remain low, so CAP is unable to activate transcription of the lac operon, regardless of the presence or absence of lactose. Upon the exhaustion of the glucose supply, cAMP levels rise, allowing CAP to activate the genes necessary for the metabolism of other food sources, including lactose if it is present.^[3]

References

1. ^ Johnston, M.; M. Carlson (1992). "Regulation of carbon and phosphate utilization", in E. W. Jones, J. R. Pringle, and J. R. Broach (ed.): The Molecular and Cellular Biology of the Yeast Saccharomyces cerevisiae, vol. 2: Gene Expression. Cold Spring Harbor Laboratory Press, 193-281. 
2. ^ Mulligan, Martin. Induction. Retrieved on 2007-01-01.
3. ^ Brown, T.A.. Transient Changes in Gene Activity. Retrieved on 2007-01-01.