Improving the Lac system for synthetic biology

The Escherichia coli lac operon is controlled by a regulatory system that has been the subject of intensive study for the past fi fty years. The system creates metabolic effi ciency by responding to the levels of environmental lactose. In the absence of lactose, the LacI protein acts as a repressor of transcription from the lac promoter. Transcription begins when lactose binds to LacI, which results in the expression of three genes involved in lactose uptake and catabolism. The lac promoter is the most commonly used promoter in the fi eld of synthetic biology. Although it is widely used, the lac promoter is known to have leaky transcription, meaning that transcription takes place even when the repressor is present and the inducer is absent. In an effort to redesign the lac promoter, we tested pLac variants that were reported to have a higher affi nity for RNA polymerase than the wild-type. We also compared three mutants of the LacI repressor that were reported to have increasing affi nity for the pLac promoter. Using GFP reporter constructs, we found that the pLacIQ1 promoter showed much higher levels of transcription than the wild-type promoter. Of the twelve combinations of promoters and repressors tested in the presence and absence of an inducer, we discovered that the wild-type LacI repressor protein with the pLacIQ1 mutant promoter is the best combination for high levels of induction and low levels of leaky transcription. Our results promise to help synthetic biologists design and build systems with tighter regulatory control. Correspondence to: A. Malcolm Campbell, Box 7118, Davidson College, Biology Dept., Davidson, NC 28035; phone: (704) 894-2692; fax: (704) 894-2512; e-mail: [email protected]. Introduction S ynthetic biology is a new fi eld of study that blends biology with mathematics, computer science and engineering. Synthetic biologists are designing and building DNA devices to alter the output of cells for important applications in medicine, energy, technology, and the environment (Baker et al., 2006). For example, synthetic biologists in Scotland designed and constructed bacteria that can visually warn people about trace amounts of arsenic in drinking water (University of Edinburgh, 2006). Other synthetic biologists at UC Berkeley reengineered microbial metabolism to produce a powerful anti-malaria medication for one tenth the cost of conventional production methods (Ro et al., 2006). Using bacteria to perform such functions is benefi cial and may soon lead to the production of carbon-neutral biofuels from waste material (Service, 2008). In order to build systems that can effi ciently produce desired outputs (like those described above), tight control of genetic circuits is required to turn the system