Novel cap analogs for in vitro synthesis of mRNAs with high translational efficiency

Synthetic analogs of the N7-methylated guanosine triphosphate cap at the 5' end of eukaryotic mRNAs and snRNAs have played an important role in understanding their splicing, intracellular transport, translation, and turnover. We report here a new series of N7-benzylated dinucleoside tetraphosphate analogs, b7Gp4G, b7m(3'-O)Gp4G, and b7m2Gp4G, that extend our knowledge of the role of the cap in translation. We used these novel analogs, along with 10 previously synthesized analogs, to explore five parameters: binding affinity to eIF4E, inhibition of cap-dependent translation in a rabbit reticulocyte lysate system, efficiency of incorporation into RNAs during in vitro transcription (% capping), orientation of the analog in the synthetic mRNA (% correct orientation), and in vitro translational efficiency of mRNAs capped with the analog. The 13 cap analogs differed in modifications of the first (distal) and second (proximal) guanine moieties, the first and second ribose moieties, and the number of phosphate residues. Among these were analogs of the naturally occurring cap m3(2,2,7)Gp3G. These compounds varied by 61-fold in affinity for eIF4E, 146-fold in inhibition of cap-dependent translation, 1.4-fold in % capping, and 5.6-fold in % correct orientation. The most stimulatory analog enhanced translation 44-fold compared with uncapped RNA. mRNAs capped with b7m2Gp4G, m7Gp3m7G, b7m(3'-OGp4G, and m7Gp4m7G were translated 2.5-, 2.6-, 2.8-, and 3.1-fold more efficiently than mRNAs capped with m7Gp3G, respectively. Relative translational efficiencies could generally be explained in terms of cap affinity for eIF4E, % capping, and % correct orientation. The measurement of all five parameters provides insight into factors that contribute to translational efficiency.