Question

In general terms, what accounts for the different preferences of $\lambda$ repressor and Cro for the three operator sites? 

   In general terms, what accounts for the different preferences of $\lambda$ repressor and Cro for the three operator sites?
Molecular Biology
Molecular Biology
Robert F. Weaver 5th Edition
Chapter 9, Problem 3 ↓

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The $\lambda$ repressor (CI) and Cro (control of repressor and others) are proteins involved in the genetic switch that determines whether the $\lambda$ bacteriophage will enter a lysogenic or lytic cycle. The $\lambda$ repressor promotes the lysogenic cycle by  Show more…

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In general terms, what accounts for the different preferences of $\lambda$ repressor and Cro for the three operator sites?
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Key Concepts

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Protein-DNA Interactions
This concept covers the ways in which proteins, particularly regulatory ones like lambda repressor and Cro, recognize and bind to specific DNA sequences. The specificity is determined largely by the structure of the protein’s DNA-binding domain and the corresponding shape and chemical features of the DNA, such as the pattern of hydrogen bond donors and acceptors along the bases.
Binding Affinity
Binding affinity refers to the strength of the interaction between a protein and its target DNA sequence. Even minor differences in DNA sequence can significantly alter the binding energy, leading to preferential binding of one regulatory protein over another. This affects the regulation of gene expression by modulating how strongly a protein binds to different operator sites.
Operator Sequence Variability
Operator sites are short segments of DNA with nucleotide sequences that vary slightly from one another. Such variability influences the precise molecular contacts that can be made with DNA-binding proteins, thereby accounting for different preferences. These differences enable proteins like lambda repressor and Cro to distinguish between otherwise similar binding sites.
Structural Complementarity
Structural complementarity refers to the way the three-dimensional shapes of a protein and its DNA binding site fit together. This concept is essential in understanding the specificity of protein-DNA interactions because it determines how effectively a protein can make contact with the DNA, which in turn affects binding affinity and specificity.
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For the following questions, consider these known facts about the role of cI protein in regulating the lysis/lysogeny decision in lambda phage: 1. Binding of cI protein to O_R1 inhibits transcription of the Cro gene. Thus, cI acts as a repressor of Cro gene transcription. 2. Interactions between cI protein bound to O_R2 and RNA polymerase bound to O_R3 (which also serves as the promoter for cI gene transcription) activate transcription of the cI gene. Thus, cI acts as an activator of cI gene transcription. 3. Transcription of the cI gene is inhibited by cI protein bound to O_R3. However, since the affinity of cI for O_R3 is weak, O_R3 is occupied by cI protein only when there is a high concentration of cI protein in the cell. E. If there is a burst of cI gene expression, which function of cI protein will be observed first? Why? F. What advantage might transcriptional inhibition confer when cI protein binds to O_R3? Why? cI protein exists as a dimer in solution and when bound to DNA (see Figure 3). cI dimers bind cooperatively to adjacent operator sites, one repressor dimer binding to each site. The term "cooperativity" refers in this case to a positively cooperative interaction in which the binding of a cI dimer to the high affinity OR1 site increases the affinity of a second dimer for the weaker OR2 site. Thus although the OR1 and OR2 sites differ in their affinities for cI by ~10-fold, two cI dimers bind simultaneously when OR1 and OR2 sites are adjacent (Figure 3). G. How might lambda phage benefit from the positively cooperative binding of cI to OR1 and OR2? H. cI proteins have two domains – the N-terminal domain, which contacts DNA, and the C-terminal domain, which mediates dimer formation and the cooperative interaction between dimers (Figure 3). What would happen to cI and Cro gene expression if the only available cI protein was truncated before the C-terminal domain, such that only the N-terminal domain remained? Why? Under normal circumstances, OR1 and OR2 are adjacent on the DNA, thus the C-terminal domains of a cI dimer bound to OR1 can interact with the C-terminal domains of a dimer bound to OR2 as shown schematically in Figure 3. Hochschild and Ptashne decided to ask what would happen to cooperative binding if the distance between OR1 and OR2 was increased.
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