|INDEX > Messenger RNA|
5.6 The 5′ end of eukaryotic mRNA is capped
|Key terms defined in this section|
|CAP (also known as CRP) is a positive
regulator protein activated by cyclic AMP. It is needed for RNA polymerase
to initiate transcription of certain (catabolite-sensitive) operons of
Transcription starts with a nucleoside triphosphate (usually a purine, A or G). The first nucleotide retains its 5′ triphosphate group and makes the usual phosphodiester bond from its 3′ position to the 5′ position of the next nucleotide. The initial sequence of the transcript can be represented as:
But when the mature mRNA is treated in vitro with enzymes that should degrade it into individual nucleotides, the 5′ end does not give rise to the expected nucleoside triphosphate. Instead it contains two nucleotides, connected by a 5′¡V5′ triphosphate linkage and also bearing methyl groups. The terminal base is always a guanine that is added to the original RNA molecule after transcription.
Addition of the 5′ terminal G is catalyzed by a nuclear enzyme, guanylyl transferase. The reaction occurs so soon after transcription has started that it is not possible to detect more than trace amounts of the original 5′ triphosphate end in the nuclear RNA. The overall reaction can be represented as a condensation between GTP and the original 5′ triphosphate terminus of the RNA. Thus
Gppp + pppApNpNp...
GpppApNpNp... + pp + p
The new G residue added to the end of the RNA is in the reverse orientation from all the other nucleotides.
Figure 5.18 The cap blocks the 5¢F end of mRNA and may be methylated at several positions.
This structure is called a cap. It is a substrate for several methylation events. Figure 5.18 shows the full structure of a cap after all possible methyl groups have been added. Types of caps are distinguished by how many of these methylations have occurred:
In a population of eukaryotic mRNAs, every molecule is capped. The proportions of the different types of cap are characteristic for a particular organism. We do not know whether the structure of a particular mRNA is invariant or can have more than one type of cap.
In addition to the methylation involved in capping, a low frequency of internal methylation occurs in the mRNA only of higher eukaryotes. This is accomplished by the generation of N6 methyladenine residues at a frequency of about one modification per 1000 bases. There are 1¡V2 methyladenines in a typical higher eukaryotic mRNA, although their presence is not obligatory, since some mRNAs do not have any (for review see 25).
|25:||Bannerjee, A. K. (1980). 5?/FONT>-terminal cap structure in eukaryotic mRNAs. Microbiol. Rev. 44, 175-205.|