Transcription and translation occur simultaneously within prokaryotic cells. During transcription, the 5' end of RNA is available for ribosomal binding. Translation commences while the 3' end is still being transcribed. In eukaryotes, by contrast, transcription and translation occur in separate compartments at different times. Conditions that allow both transcription and eukaryotic translation to occur in a single tube may be suboptimal for one or both reactions.
Linked systems are two-step procedures.
First, transcription is catalyzed with an appropriate polymerase. Second, a cell-free lysate system, usually eukaryotic, is used to effect translation. Because the transcription and translation reactions are separate in linked systems, each can be optimized. Whether planning to use a conventional, linked, or coupled system, you must first consider which type of translation lysate to employ. The most frequently used cell-free translation systems consist of crude extracts from E. coli , wheat germ, or rabbit reticulocytes along with supplements required for efficient translation.