The 5´terminal m7G cap present on most eukaryotic mRNAs promotes translation in vitro at the initiation level.

For most RNAs, elimination of the cap structure causes a loss of stability, especially against exonuclease degradation, and a decrease in the formation of the initiation complex of mRNAs for protein synthesis. Certain prokaryotic mRNAs containing a 5´ terminal cap structure are translated as efficiently as or more efficiently than eukaryotic mRNAs in a eukaryotic cell-free protein synthesizing system. Also a cap requirement has been observed for splicing eukaryotic substrate RNAs. A method for efficient in vitro synthesis of capped RNA using E. coli RNA polymerase primed with m7G(5´ )ppp(5´ )G or m7G(5´ )ppp(5´ )A has been developed by Contreas et al.