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Team:Edinburgh/Project
From 2010.igem.org
BRIDGE: The concept
BRIDGE stands for BioBrick Recombination In Direct Genomic Editing. It is an alternative method of inserting BioBricks into the genome using homologous recombination instead of restriction digestion, and has the added of bonus of not leaving a marker behind.
Image: Appl Environ Microbiol. 2008 July; 74(13): 4241–4245 (Fig. 1)
The first step of BRIDGE requires the deletion of existing DNA (probably a non-coding piece or a non-essential gene) to introduce a construct of two genes; one an antibiotic resistance gene, the other sacB, which prevents the host from growing on sucrose. After the first step we can select for cells which have taken up the construct by growing them on the relevant antibiotic.
The second step involves swapping the construct for another piece of DNA (e.g. a BioBrick construct). After this we can select for those with the new gene by growing the cells on sucrose.
BRIDGE: The advantages
BRIDGE has a significant advantage over the current method of BioBrick insertion. For one, it is vector independent - whole PCR constructs can be inserted directly into the genome in two steps in under a week, compared to the lengthy process of vector digestion and ligation required with normal BioBricks.
The other major advantage is that it will not leave a lasting marker in the genome. With most BioBricks we have to leave a marker (antibiotic resistance, GFP, etc) in our constructs so that we can guarantee their presence. This becomes an issue, a) when you want to use the organism in an industrial or environmental capacity, and b) when you want to insert multiple constructs (there is only a limited number of markers out there). With this system, the markers are removed every time you insert a new gene, so they can be used again and again indefinitely. You could essentially replace the entire genome with new genes.
Our Project
Our BRIDGE construct will contain chloramphenicol resistance (cat) and sacB. Both it and the desired gene will be inserted by homologous recombination using the lambda red system. For this we will need up and down-stream sequences of genes which we wish to replace.
To prove the principle of BRIDGE we will remove a non-essential, constitutively expressed gene from the E. coli genome and replace it with a well known marker, such as GFP. We also have several genes from a past project idea which we could delete to increase fatty acid synthesis, and further genes we could introduce which will result in the production of long chain alkenes from the excess fatty acids. This is not useful for our current project but it is a nice way to demonstrate the effectiveness of BRIDGE.
Eventually, BRIDGE will be used to introduce whole light producer-sensor constructs (we hope).
