MCAT Biological and Biochemical Foundations of Living Systems: Passage 15 — Flashcards

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An operon is a cluster of contiguous genes controlled by a single promoter and a regulatory element called the operator, which usually lies downstream from the promoter. The genes in an operon typically have related functions. The classic example of an operon, and the first to be characterized, is the lac operon in E. coli. The lac operon is composed of a promoter element (P); three operators (Os), termed O1, O2, and O3; and three genes, lacZ, lacY, and lacA, which encode, respectively, β-galactosidase, β-galactoside permease, and β-galactoside transacetylase. Outside of the operon, the I gene encodes the lac repressor; in the absence of lactose, the lac repressor, as a tetrameric complex, binds the operators, blocking RNA polymerase transcription of lacZ, lacY, and lacA.

As a result, these genes are expressed at low levels, giving rise to a baseline amount of enzymatic activity. However, when lactose is present outside the cell, the baseline level of β-galactoside permease pumps lactose into the cell, and β-galactosidase converts it to allolactose. Allolactose binds the repressor, resulting in its release from the operators. In this way, the lac operon is controlled by a positive feedback: lactose increases expression of lacZ and lacY, which in turn increases the uptake of lactose into cells. However, other energy sources also affect the induction of the lac operon. In the presence of a glucose, which is a more efficient energy source than lactose, the lac operon is repressed through a regulator protein called catabolite activator protein (CAP) and cyclic AMP (cAMP). The components of the lac operon were all originally identified through mutations, such as in the I gene (I–), the promoter (P–), and the operator elements (O–) that affected expression of lacZ, lacY, and lacA.