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A dosimeter is a device which measures the amount of ionizing radiation an individual or object has received over time. With the renewed interest in nuclear energy in response to dwindling fossil fuel reserves, concerns over nuclear waste storage and disposal, and fear of radiological terrorism, the need for a cheap dosimeter is clear. A biological system that acts as a dosimeter would be a cheap and readily producible first response indicator that could be interpreted without training or calculation.

The Bio-Dosimeter

In order to monitor radiation dose the bio-dosimeter would need to sense incoming ionizing radiation, a trait that organisms do not normally possess. However, the genetic damage that makes radiation dangerous is also readily monitored by several highly specific systems in nature. One such example of this is the Lambda Bacteriophage, which switches from the dormant lysogenic state to the active lytic state in response to genetic damage in its bacterial host. Our project uses the lambda phage genome as a radiation biosensor as it has been extensively characterized and has tight control over gene expression.

Lambda Bacteriophage

The lambda bacteriophage controls switching between the lysogenic/lytic states once lysogeny has been estabilished through a bi-directional promoter (PR, PRM) with three binding sites with different affinities for repressor (Cl) and activator (Cro). The lysogen normally maintains lysogeny through a feedback loop with repressor Cl that simultaneously maintains Cl concentration within a set range and represses the Cro activator by activating the PRM promoter which expresses Cl and repressing the PR promoter which expresses Cro. However, in response to genetic damage the lambda phage exploits the host’s SOS response to cleave the repressor. As Cl concentration declines, the stronger PR promoter begins to express Cro, which competes with Cl for binding sites and represses Cl’s production.

Using the Bacteriophage as a Dosimeter

By taking the PR, PRM bidirectional promoter and placing desired response proteins such as GFP downstream of the activator Cro, the operator acts as a tightly regulated dosimeter with little chance of accidental false positives.