Unformatted text preview: nes and Gene Expression, R.L. Rodriguez 18 Capping with Guanylyl Transferase γ P β P α α γ
β α P P
P γ β P β γ α BIS101001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez ©2013
BIS101001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez 19 Sequence of Capping Reaction
Step 1: The gamma phosphate of the base at the 5' end of the hnRNA is
removed by an enzyme called nucleotide phosphohydrolase.
removed Step 2: A molecule of GTP condenses with the end of the RNA, catalyzed
by the enzyme guanylyl transferase. The gamma and beta phosphates of
the GTP are lost as PPi during this reaction. The resulting linkage is 5' to
5'' as indicated in all of the structures shown above.
5 Step 3: The guanine residue of the cap is methylated at the 7 position of
the base, catalyzed by a methyl transferase and using as a source of
methyl groups S-adenosyl methionine (SAM).
methyl Step 4: Sometimes, methyl transferase will add methyl groups to the
2'-OH of the ribose sugars of the first or second base in the RNA, again
using SAM as a methyl donor. These structures are called Cap 1 and
Cap 2, respectively.
Cap BIS101001, Spring 2013—Genes and Gene Expression, R.L. Rodrig...
View Full Document