Enabling synthetic biology with an expanded library of engineered orthogonal genetic logic gates and switches

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An important goal of synthetic biology is the rational design and predictable implementation of synthetic gene circuits using standardised and interchangeable parts to program cellular behaviour. However, unlike electronic digital circuits, the components in a biological circuit are not connected by wires with physical insulation, and the flow of biological information has to depend on their specific chemical interactions to avoid cross talk. As a result, the same genetic part may not be used twice in one integrated system to prevent the potential unintended interactions between them. Therefore, orthogonal parts and modules are necessary for the compatibility and scalable design of large gene circuits comprising many components. Orthogonality implies that the newly added parts and modules should not cross-talk with those present in the engineered biological systems as well as the host genetic background. Most of the gene circuits constructed so far are small scale systems that have been constructed by costly and inefficient 'trial-and-error' methods with very limited parts. For example, it has taken almost 12 years to progress from the first 3-gene toggle switch to the so far largest constructed 11-gene 4-input AND logic gate in a single cell. A hard truth behind this slowness is that the engineering of complex circuits in living cells is currently limited by the availability of well-characterised and orthogonal (non cross-talk) genetic regulatory building blocks. Hence, an urgent need in synthetic biology is to expand the currently limited toolbox of biological parts with many functional orthogonal elements to scale up our capacity for building large and complex circuits.Nevertheless, it remains a big foundational challenge to expand the range of available orthogonal components in the synthetic biology toolbox. This project aims to address this challenge by developing two novel scalable tools to engineer an expanded library of versatile orthogonal genetic building blocks. In particular, we will build a library of modular and orthogonal genetic NAND and NOR logic gates; these are universal logic gates and their combinations can be used to accomplish any arbitrary complex Boolean logic operations, providing a powerful scalable method for cellular process control. Further, we will create multi-layer genetic programs from different permutations of these engineered logic gates to demonstrate the potential for composing high-order signal processing and transcriptional control functions in a single cell. For example, the engineered genetic programs will be used to implement a high level logic computing device - 1 bit full adder that intake three chemical inputs in specified logic manners to produce two optical outputs. In addition, we will demonstrate that large complex transcriptional control programs can be implemented in a microbial cell factory to precisely and rapidly tune gene expression profiles within the biosynthesis pathway of a high value chemical (violacein).The engineered scalable tools from this study will increase significantly the number of orthogonal control elements, gates and wires in the limited toolbox of synthetic biology, leading to large-scale complex genetic control programs attainable to program advanced behaviours in cells. The successful outcome will lead to a number of applications expected in the biotechnology industry (high gain), and will be of enormous benefit to researchers not only in the synthetic biology and but also in bioengineering communities and those in the biotechnology industry.