W W W W W W W W W W Nx

Early lytic growth

Repressor promoter cro promoter or3 or2 or1 i-1 i-1 r

RNA polymerase mura

Repressor promoter cro promoter

Figure 39-7. Configuration of the switch is shown at four stages of lambda's life cycle. The lysogenic pathway (in which the virus remains dormant as a prophage) is selected when a repressor dimer binds to OR1, thereby making it likely that OR2 will be filled immediately by another dimer. In the prophage (top), the repressor dimers bound at OR1 and Or2 prevent RNA polymerase from binding to the rightward promoter and so block the synthesis of Cro (negative control). The repressors also enhance the binding of polymerase to the leftward promoter (positive control), with the result that the repressor gene is transcribed into RNA (wavy line) and more repressor is synthesized, maintaining the lysogenic state. The prophage is induced when ultraviolet radiation activates the protease recA, which cleaves repressor monomers. The equilibrium of free monomers, free dimers, and bound dimers is thereby shifted, and dimers leave the operator sites. RNA polymerase is no longer encouraged to bind to the leftward promoter, so that repressor is no longer synthesized. As induction proceeds, all the operator sites become vacant, and so polymerase can bind to the rightward promoter and Cro is synthesized. During early lytic growth, a single Cro dimer binds to Or3 shaded circles, the site for which it has the highest affinity. Consequently, RNA polymerase cannot bind to the leftward promoter, but the rightward promoter remains accessible. Polymerase continues to bind there, transcribing cro and other early lytic genes. Lytic growth ensues. (Reproduced, with permission, from Ptashne M, Johnson AD, Pabo CO: A genetic switch in a bacterial virus. Sci Am [Nov] 1982;247:128.)

To date, this system provides the best understanding of the molecular events involved in gene regulation.

Detailed analysis of the lambda repressor led to the important concept that transcription regulatory proteins have several functional domains. For example, lambda repressor binds to DNA with high affinity. Repressor monomers form dimers, dimers interact with each other, and repressor interacts with RNA polymerase. The protein-DNA interface and the three proteinprotein interfaces all involve separate and distinct domains of the repressor molecule. As will be noted below (see Figure 39-17), this is a characteristic shared by most (perhaps all) molecules that regulate transcription.

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