A modified guanine nucleotide terminating eukaryotic mRNA molecules. The cap is introduced after transcription by linking the 5′ end of a guanine nucleotide to the 5′ terminal base of the mRNA and adding a methyl group to position 7 of this terminal guanine. The addition of the terminal guanine is catalyzed by the enzyme guanylyl transferase. Another enzyme, guanine-7-methyl transferase, adds a methyl group to the 7 position of the terminal guanine. Unicellular eukaryotes have a cap with this single methyl group (cap 0). The predominant form of the cap in muticellular eukaryotes (cap 1) has another methyl group added to the next base at the 2′- o position by the enzyme 2′- o-methyl transferase. More rarely, a methyl group is also added to the 2′ -o position of the third base, creating cap 2 type. Capping occurs shortly after the initiation of transcription and precedes all excision and splicing events. The function of the cap is not known, bu it may protect the mRNA from degradation by nucleases or provide a ribosome binding site. See posttranscriptional processing.
Subjects: Genetics and Genomics.