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Review
. 2008 Aug 6;5 Suppl 1(Suppl 1):S29-39.
doi: 10.1098/rsif.2008.0086.focus.

Quantitative approaches to the study of bistability in the lac operon of Escherichia coli

Moisés Santillán  1 , Michael C Mackey
Affiliations
Review

Quantitative approaches to the study of bistability in the lac operon of Escherichia coli

Moisés Santillán et al. J R Soc Interface. .

Abstract

In this paper, the history and importance of the lac operon in the development of molecular and systems biology are briefly reviewed. We start by presenting a description of the regulatory mechanisms in this operon, taking into account the most recent discoveries. Then we offer a survey of the history of the lac operon, including the discovery of its main elements and the subsequent influence on the development of molecular and systems biology. Next the bistable behaviour of the operon is discussed, both with respect to its discovery and its molecular origin. A review of the literature in which this bistable phenomenon has been studied from a mathematical modelling viewpoint is then given. We conclude with some brief remarks.

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Figures

Figure 1
Figure 1
Typical diauxic growth curve. Note the existence of two different exponential growth phases, separated by a short interval in which the culture does not grow. The first (second) phase corresponds to the bacterial culture feeding on glucose (lactose), while the interval with no growth corresponds to the time the bacteria need to turn on the genes needed to metabolize lactose after glucose exhaustion.
Figure 2
Figure 2
(a) Schematic of the regulatory elements located in lac operon DNA. P denotes the promoter, O1, O2 and O3 correspond to the three operators (repressor-binding sites), and C is the binding site for the cAMP–CRP complex. The different ways in which a repressor molecule can interact with the operator sites are represented in b, c, d and e. Namely, a free repressor molecule (b), one with a single subunit bound by allolactose (d) or one with the two subunits in the same side bound by allolactose (e) can bind a single operator. Moreover, a free repressor molecule can bind two different operators simultaneously (c). Figure adapted from Santillán (2008).
Figure 3
Figure 3
Schematic of the lac operon regulatory mechanisms. This operon consists of genes lacZ, lacY and lacA. Protein LacY is a permease that transports external lactose into the cell. Protein LacZ polymerizes into a homotetramer named β-galactosidase. This enzyme transforms internal lactose (Lac) to allolactose (Allo) or to glucose and galactose (Gal). It also converts allolactose to glucose and galactose. Allolactose can bind to the repressor (R) inhibiting it. When not bound by allolactose, R can bind to a specific site upstream of the operon structural genes and thus avoid transcription initiation. External glucose inhibits the production of cAMP that, when bound to protein CRP to form complex CAP, acts as an activator of the lac operon. External glucose also inhibits lactose uptake by permease proteins. Figure adapted from Santillán et al. (2007).
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