# USTC/OperatorComposition

### From 2007.igem.org

Generally, Lac repressor bind to its operator in the form of dimer. Almost all the according operators in nature are of approximate reverse symmetric structure, that is, each half of the operators consists of about 10 nucleotides and receives the binding of a repressor monomer. It has been reported that the perfectly symmetric operator binds native Lac repressor most tightly. Based on this fact, an idea was proposed that the binding affinity of a non-symmetric operator may be able to be systematically reduced, exempt from the harm on the binding specificity of the repressor-operator pairs.

Five promoters have been synthesized and tested:

- O11, the perfectly symmetric operator
- O1wt1, the native lac operator at +11 site of Plac promoter
- O16, the right half of O11 replaced with a sequence which binds Lac repressor weakly, but has the same pattern as O11.
- O17, the right half of O11 replaced with a random sequence
- NUL, the positive control

After a series of experiments, we came to the conclusion that the method of substituting the right half of the symmetric operator can systematically reduce the binding affinity. Ox6, which means the right half of a symmetric operator is substituted with a weak operator sequence with the same pattern, can be used as a "weak" operator. At the same time, we found that the pattern Ox7 is weaker than Ox6, and the affinity of Ox7 is too low for a logic gate.