THE SCIENTIST STAFFCELLULAR ENGINEERING IN CONTEXT
In contrast to other engineering disciplines—in which static elements are snapped into an electrical circuit, for example, or sturdy pieces of lumber are assembled into a larger structure—designing circuits inside living cells is messy business. In the hypothetical example depicted here, genetic components, such as transcription factors (TFs) and green fluorescent protein (GFP) reporters, are first integrated into the yeast genome.1 The addition of a chemical inducer initiates expression of the first component in the circuit, TF12, which in turn activates the expression of a second transcription factor, TF23. Finally, TF2 initiates the expression of GFP4, completing the cascade. In order for the circuit to function properly, the TFs must wade through the crowded and chaotic environment of the cell to do their jobs. Once expressed, they enter the cytoplasm for translation into proteins where they are bound to encounter hundreds or thousands of proteins or other cellular materials5. This risks not only disrupting the intended circuit, but also affecting basic cellular function, possibly disrupting important signaling pathways, for example. Once a transcription factor translocates into the nucleus, it must then find its target sequence among the millions of base pairs of the yeast genome—a challenging combinatorial task. This often leads to nonspecific binding within the genome6, which can perturb off-target genes and once againjeopardize the engineered circuit and cause undesired cellular effects.