Cells are, at a fundamental level, protein-production facilities. So naturally, when researchers need to make some particular protein, they should let the cells do the work for them. But living cells are not terribly good at making exogenous proteins; some proteins are toxic, while others are degraded or simply clumped into insoluble aggregates called inclusion bodies.

These days, scientists sometimes take a minimalist approach, throwing out the cell and its contaminating components altogether and keeping only what they need for transcription and translation. Such in vitro extracts allow protein production from linear or circular DNA, in quantities large enough to purify and analyze. This tack also provides a way to incorporate modified amino acids into the protein without poisoning the cell, or to express mutated gene products without lengthy cloning steps.1 But there are downsides, too. Most in...

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