Question

In your own words, explain why it is necessary for the \(c I\) gene to have two promoters. What would happen if it had only \(P_{\mathrm{RE}}\) ?

Short answer

The \(c I\) gene requires two promoters, \(P_{\mathrm{RM}}\) and \(P_{\mathrm{RE}}\), for accurate and regulated gene expression. This dual promoter system enables the efficient switching between the lytic and lysogenic cycle of the lambda phage. If the gene had only \(P_{\mathrm{RE}}\), it might not be able to switch effectively between these cycles as expression is dependent on a lower level of CI proteins which can be inefficient without the input from \(P_{\mathrm{RM}}\).

Step-by-step solution

Understand the function of promoters in gene expression

Initially, it's important to understand that promoters are DNA sequences located in the 5' region upstream from the transcription start site. They are responsible for the regulation of gene expression. The promoters recognized by the RNA polymerase are extremely important for correct temporal and spatial expression of the gene.

Explain the necessity for the \(c I\) gene to have two promoters

In the case of the \(c I\) gene, it has two promoters, \(P_{\mathrm{RM}}\) and \(P_{\mathrm{RE}}\), which contribute to its controlled expression. Having two promoters allows a finer control over gene expression. While \(P_{\mathrm{RM}}\) primarily drives \(c I\) gene expression, \(P_{\mathrm{RE}}\) is responsible for the recruitment of RNA polymerase when the CI repressor protein level is low, which is highly dependent on the multiplicity of infection. Therefore, this dual promoter system contributes to the efficient switch between the lytic and lysogenic cycle of lambda phage.

Analyze the outcome if the \(c I\) gene had only \(P_{\mathrm{RE}}\)

If the \(c I\) gene had only \(P_{\mathrm{RE}}\), it could fail to express at the right time, as \(P_{\mathrm{RE}}\) is dependent on low levels of CI repressor proteins. Without \(P_{\mathrm{RM}}\) to drive gene expression, the DNA might not be able to switch from the lytic to lysogenic cycle effectively, leading to the inability of lambda phage to switch into a lysogenic cycle.