This week I attended a cell-free synthetic biology workshop at Dr. Richard Murray's lab at Caltech. We learned how they make cell-free extracts for their experiments and made a batch ourselves. Cell-free extracts start with whole cells (in most cases E.coli), remove the cell membranes, centrifuge out genome, and dialyze to remove salts. What is left is a pared down set of proteins that are used to make proteins from any desired gene. Salts, nucleotides, amino acids, and energy molecules are added back to sustain protein synthesis. The goal of their work is to make cell-free systems more predictable and reliable. They mostly use cell-free extract to test genetic circuits before putting them in cells.
Their protocol is published in the Journal of Visualized Experiments (JOVE). Sun, Zachary Z., et al. "Protocols for implementing an Escherichia coli based TX-TL cell-free expression system for synthetic biology." Journal of visualized experiments: JoVE 79 (2013).