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#mRNA cap

2 public questions tagged with this topic.

Processing of mRNA, rRNA, tRNA

5' cap is linked to mRNA via

Linkage between seven methylguanosine cap and first transcribed nucleotide involves unusual five prime to five prime triphosphate bridge, chemically distinct from standard three prime to five prime phosphodiester backbone connecting nucleotides within chain. Reaction catalyzed by guanylyltransferase creates G-five prime ppp five prime-N arrangement where three phosphate groups link inverted guanosine to RNA, blocking free five prime phosphate. Such inverted connectivity makes RNA resistant to five prime to three prime exonucleases Xrn1 and Xrn2 because exonucleases require free monophosphate entry site, thus protecting messenger RNA stability in both nucleus and cytoplasm.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 10: 5'-5' triphosphate bond of cap; Alberts et al., Mechanism of capping

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Processing of mRNA, rRNA, tRNA

The 5' cap contains which modified nucleotide?

Five prime cap of eukaryotic messenger RNA contains modified nucleotide seven methylguanosine, formed after addition of guanosine not encoded by DNA. Guanine base is methylated at N7 position by cap methyltransferase RNMT using S-adenosylmethionine SAM as methyl donor, creating positively charged quaternary ammonium essential for specific recognition by eukaryotic initiation factor eIF4E during translation. Additional 2'-O methylations of first nucleotide creating cap one and second nucleotide creating cap two occur via CMTR1 and CMTR2 methyltransferases in higher eukaryotes enhancing innate immune evasion and translation efficiency.

Ref: Berg et al., Biochemistry, 9th ed., Chapter 28: 5' cap contains 7-methylguanosine; Alberts 7th ed., Cap methyltransferase mechanism

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