Synthetic Biology Bicistronic Designs Support Gene Expression Equally Well in vitro and in vivo

Synthetic biology integrates molecular biology tools and an engineering mindset to address challenges in medicine, agriculture, bioremediation, and biomanufacturing. A persistent problem in synthetic biology has been designing genetic circuits that produce predictable levels of protein. In 2013, Mutalik and colleagues developed bicistronic designs (BCDs) that make protein production more predicable in bacterial cells (in vivo). With the growing interest in producing proteins outside of cells (in vitro), we wanted to know if BCDs would work as predictably in cell-free protein synthesis (CFPS) as they do in E. coli cells. We tested 20 BCDs in CFPS and found they performed very similarly in vitro and in vivo. As a step toward developing methods for protein production in artificial cells, we also tested 3 BCDs inside nanoliter-scaled microfluidic droplets. The BCDs worked well in the microfluidic droplets, but their relative protein production levels were not as predictable as expected. These results suggest that the conditions under which gene expression happens in droplets result in a different relationship between genetic control elements such as BCDs and protein production than exists in batch CFPS or in cells.