The Imperial iGEM 2008 team faces the task of working with a chassis that has been rarely used - and never characterised - in the competition to date. While the B. subtilis chassis offers us many advantages, working from the ground up presents many challenges.
Our cloning strategy is complex. In order to build the required constructs for our final product, we need to build, test and characterise intermediary parts and devices that will lead to the final system. The diagram below shows the critical pathway for our cloning strategy with a large number of closely-linked steps.
A summary of the aims of each phase and constructs to be produced within each phase, is given below:
Phase 1; Constitutive Promoter Testing
Testing and characterisation of constitutive promoters. We will test 4 combinations of 2 promoters and 2 RBSs to characterise them. An antibiotic resistance cassette is placed at the 5' end of the construct, to prevent any readthrough by the native trancriptases from reaching the regulated biobricks.
Phase 2; Inducible Promoter Testing
Testing and characterisation of inducible promoters; those marked with a 'C' are chemically-inducible and those marked with an 'L' are light-inducible. RFP is used instead of GFP as a quantifiable output as ytvA responds to blue light - GFP may cause positive feedback. 'Rep' genes encode a repressor for the chemically-inducible promoters to stop leaky expression.
Phase 3; Clutch and Biomaterial Characterisation
Testing and characterisation of the clutch (epsE) and biomaterial synthesis (SB - signal sequence & biomaterial) using a chemically-inducible promoter. The promoter is otherwise constitutively repressed by Rep.
Phase 4; Device Characterisation
Combination of light induction and EpsE/biomaterial expression. The clutch (EpsE) and biomaterial are now under control of the light-inducible activator sigma B (via YtvA).
Final Construct; System Characterisation
Combination of light sensing and light-induced expression of epsE and biomaterial. Each gene has its own promoter/RBS pair because in B. subtilis, it has been shown that levels of expression decreases as one moves along an operon.