[Gene expression in cell-free systems on a preparative scale]

In vivo expression of foreign or synthetic genes can be subject to certain restrictions such as protein aggregation, degradation and toxicity. Conventional in vitro systems can overcome these problems, but in turn suffer from other limitations, in particular short life-time and low protein yield. In this review, two types of gene expression system are described. Both are based on the novel concept of the enhanced expression from cell-free extracts where incubation is performed in the continuous flow of a feeding solution rather than in a fixed volume of a test-tube. The first type makes use of cell-free translation of mRNA templates in either prokaryotic or eukaryotic cell lysates. The second utilizes the coupled transcription--translation of DNA templates, with genes transcribed by either endogenous or bacteriophage RNA polymerases. In both systems, translation can be carried out over tens or hundreds of hours resulting in high protein yields.