Proc Natl Acad Sci U S A. 2017 Jul 18;114(29):7659-7664. doi:10.1073/pnas.1704006114

Zhang W, Tam CP, Walton T, Fahrenbach AC, Birrane G, Szostak JW.

Insight into the mechanism of nonenzymatic RNA primer extension from the structure of an RNA-GpppG complex.

« The formation of two Watson–Crick base pairs leads to a much higher affinity of GpppG than GMP for a CC template (Kd of ∼0.2 mM vs. ∼20 mM, respectively). » « The tight binding of GpppG to a CC template suggests that GpppG might be an ideal primer for the initiation of template copying by primer extension. The resulting RNAs would begin with a 5ʹ cap-like GpppG moiety, suggesting a potential evolutionary origin for the eukaryotic mRNA 5ʹ-cap structure. »