5'-Кэп мРНК: структура, функции и модификации

Specially altered nucleotide on the 5' end of pre mRNA
In molecular biology, the five prime cap (5′ cap) is a specially altered nucleotide on the 5′ end of some primary transcripts such as precursor messenger RNA. This process, known as mRNA capping, is highly regulated and vital in the creation of stable and mature messenger RNA able to undergo translation during protein synthesis. Mitochondrial mRNA and chloroplastic mRNA are not capped.

In eukaryotes, the 5′ cap (cap 0), found on the 5′ end of an mRNA molecule, consists of a guanine nucleotide connected to mRNA via an unusual 5′ to 5′ triphosphate linkage. This guanosine is methylated on the 7 position directly after capping in vivo by a methyltransferase. It is referred to as a 7 methylguanylate cap, abbreviated m7G. In multicellular eukaryotes and some viruses, further modifications exist, including the methylation of the 2′ hydroxy groups of the first 2 ribose sugars of the 5′ end of the mRNA. cap 1 has a methylated 2′ hydroxy group on the first ribose sugar, while cap 2 has methylated 2′ hydroxy groups on the first two ribose sugars, shown on the right. The 5′ cap is chemically similar to the 3′ end of an RNA molecule (the 5′ carbon of the cap ribose is bonded, and the 3′ unbonded). This provides significant resistance to 5′ exonucleases. Small nuclear RNAs contain unique 5′ caps. Sm class snRNAs are found with 5′ trimethylguanosine caps, while Lsm class snRNAs are found with 5′ monomethylphosphate caps. In bacteria, and potentially also in higher organisms, some RNAs are capped with NAD+, NADH, or 3′ dephospho coenzyme A. In all organisms, mRNA molecules can be decapped in a process known as messenger RNA decapping.