Promoters determine when your circuit should be transcribed into RNA which is then translated into protein. In other words, the promoter can be thought of as a switch which can be switched on and off in different circumstances; when the switch is on the gene is expressed and the desired protein is produced and when the switch is off the gene isn’t expressed.

This is a constitutive promoter which means that it will always express the gene regardless of the circumstances. This promoter is called J23100 and is part of a family of constitutive promoters which are all very similar. Below is a graph comparing the relative strength of the promoters in the family when paired with the red fluorescent protein gene; as you can see, J23100 is the strongest promoter in the family.

strong promoter graph

This promoter only switches on in the presence on Cd2+ ions. Below is an image of bacteria carrying the promoter as well as the red fluorescent protein gene in varying metal ion solutions. As you can see, even other heavy metals with 2+ charge can’t activate the promoter.

Cd promoter specificity
Cadmium serves no biological purpose, is very toxic and is classed as a carcinogen. Inhaling or ingesting any significant amount of cadmium can cause immediate poisoning and damage to the liver and kidneys.

Symptoms of cadmium poisoning can include softening of bones and loss of bone mineral density and in extreme cases this can result in body weight being enough to cause fractures. The kidney’s also permanently lose their ability to remove acids from the blood which results in an increase in phosphate levels causing muscle weakness. You can learn more about the health effects of cadmium here.

Below is a graph showing how effective the promoter is at different concentrations of Cd2+. It shows that the promoter requires a minimum threshold between 10-8mol/L and 3*10-8mol/L.

cadmium graph

This promoter only activates in the presence of Cu2+ ions. This genetic sequence was created and used by a team from Saint-Petersburg to construct a copper biosensor. You can find a presentation they gave about the biosensor here.

copper sensitive rfp set up

Copper is an important micronutrient and is crucial for proper growth and the maintenance of bones and many organs including the brain and heart. Copper is so important for growth that fetuses accumulate copper in the liver, resulting in a copper concentration four times greater than that in an adult, which is then released slowly after birth to make up for the lack of copper in breast milk.

The body tries to keep copper concentration in a state of homeostasis (constantly the same). It does this by excreting excess copper in bile and releasing copper from storage in the liver when the copper concentration gets low. There are, however, genetic diseases which can cause the body’s homeostatic processes to be ineffective such as Menkes syndrome and Wilson disease. You can learn more about how the body uses copper and the affects of copper deficiency here.

This promoter is different from the other two metal sensitive promoters; the iron promoter decreases the expression of your chosen gene. In other words, if you were to match this with the red fluorescent protein gene the bacteria would be less fluorescent in the presence of iron ions. In nature, this kind of promoter is used to maintain  homeostatic systems.

Iron is important for many biological processes in all living organisms. The most notable use of iron in the body is probably in hemoglobin, a protein found in the blood which is used to carry oxygen around the body. Despite iron being so important in the body, we don’t tend to get free iron ions in the body. Iron’s transport around the body is carefully regulated by proteins such as transferrin. This is because iron ions have high potential for biological toxicity. This is because iron ions in the blood can react with peroxides which produces free radicals. Free radicals can cause damage to proteins and DNA.