RNA methyltransferases involved in 5' cap biosynthesis

Byszewska, Magdalena; Smietanski, Miroslaw; Purta, Elzbieta; Bujnicki, Janusz M

The most typical and widely studied cap modification comprises the addition of an N 7 -methylguanosine (m 7 G) linked via an inverted 5 0 -5 0 triphosphate bridge to the 5 0 -terminal nucleoside of the transcript. 1 This structure termed cap0 is a characteristic feature of transcripts that are produced by RNA polymerase II, such as messenger RNAs (mRNAs) of all eukaryotic organisms and many viral RNAs. It is typically introduced in sequential steps: (1) hydrolysis of 5 0 g-phosphate of a nascent pre-mRNA to generate a 5 0 diphosphate mRNA end; (2) transfer of a guanine monophosphate nucleoside; and (3) methylation of the guanine at the N 7 position. The cap0 structure was shown to be essential for cell growth of Saccharomyces cerevisiae 2 and survival of mammalian cells; 3 it is critical for mRNA interactions with many nuclear and cytoplasmic proteins and has multiple important roles in gene expression, including enhancement of RNA stability, splicing, nucleocytoplasmic transport, and translation initiation. 4, 5 Enzymes responsible for cap0 formation have been well characterized in many organisms and viruses.