Nucleotides have been called the building blocks of life, but as you’ve seen it’s not trivial to build something with them. Recall you spent nearly three weeks truncating the gene for GFP, and your efforts to rationally modify an enzyme yielded a complex mixture of proteins and results. If one reasonable definition of a biological engineer is someone who builds things from biological materials, then let’s get building. We’ll start with an existing and clever design that uses bacteria as the pixels in a photograph. The bacterial cells have been engineered to respond to light, churning out a familiar enzyme (beta-gal) to turn the media black in the dark. You’ll be using this system to take black and white pictures, identifying experimental changes that can affect the operation of system. You will also invert the logic in the existing design and add a red-fluorescent protein in the readout to generate two-color pictures. Finally you’ll use quantitative PCR to characterize the simple or complex circuit, cataloging your findings at the Registry of Standard Biological Parts for future biological engineers to “build” on.