The initial processing reaction for premRNA involves the addition of a 5′-cap. This cap is composed of a 7-methylguanosine that is attached to the mRNA's 5′terminal end through a unique 5′→5′-triphosphate linkage, which is resistant to most nucleases. To create the cap, the γ phosphoryl group from the premRNA's 5′-triphosphate must be removed. Then, guanosine monophosphate is added to the premRNA by the nuclear enzyme guanylyltransferase, using guanosine triphosphate as the source. Methylation of the terminal guanine occurs in the cytosol and is catalyzed by guanine-7-methyltransferase, with the methyl group coming from S-Adenosylmethionine. Additional methylation steps may also occur. This addition of the 7-methylguanosine cap helps stabilize the mRNA and allows for efficient initiation of translation.

The next step is the addition of a 3′-poly-A tail. Most eukaryotic mRNA (except for histones) have a chain of 40–250 adenylates attached to their 3′-end. This poly-A tail is not transcribed from the DNA but is instead added by the nuclear enzyme polyadenylate polymerase, using ATP as the substrate. The pre-mRNA is cleaved downstream of the polyadenylation signal sequence (AAUAAA), which is located near the 3′-end of the RNA. The poly-A tail is then added to the newly created 3′-end. The addition of the poly-A tail terminates eukaryotic transcription. The poly-A tail helps stabilize the mRNA, aids in its exit from the nucleus, and assists in translation. Once the mRNA enters the cytosol, the poly-A tail gradually shortens.

The final step in the maturation of eukaryotic mRNA is splicing. This process involves the removal of RNA sequences called introns, which do not code for protein, from the primary transcript. The remaining coding sequences, called exons, are then joined together to form the mature mRNA. The molecular complex responsible for these tasks is known as the spliceosome. Some eukaryotic primary transcripts do not contain any introns, while others contain a few or even over 50 introns that need to be removed. To mediate splicing, uracil-rich small nuclear RNA (snRNA) forms five small nuclear ribonucleoprotein particles (snRNP), referred to as U1, U2, U4, U5, and U6. These snRNP particles, along with multiple proteins, facilitate the removal of introns by forming base pairs with the consensus sequences at each end of the intron. It's important to note that in systemic lupus erythematosus, an autoimmune disease, patients produce antibodies against their own nuclear proteins, such as snRNP.
Addition of a 5′-cap: This is the first of the processing reactions for premRNA (Fig. 31.17). The cap is a 7-methylguanosine attached to the 5′terminal end of the mRNA through an unusual 5′→5′-triphosphate linkage that is resistant to most nucleases. Creation of the cap requires removal of the γ phosphoryl group from the 5′-triphosphate of the premRNA, followed by addition of guanosine monophosphate (from guanosine triphosphate) by the nuclear enzyme guanylyltransferase. Methylation of this terminal guanine occurs in the cytosol and is catalyzed by guanine-7-methyltransferase. S-Adenosylmethionine is the source of the methyl group (see p. 263). Additional methylation steps may occur. The addition of this 7-methylguanosine cap helps stabilize the mRNA and permits efficient initiation of translation (see p. 455).
2.
Addition of a 3′-poly-A tail: Most eukaryotic mRNA (with several exceptions, including those for the histones) have a chain of 40–250 adenylates (adenosine monophosphates) attached to the 3′-end (see Fig. 31.17). This poly-A tail is not transcribed from the DNA but rather is added by the nuclear enzyme, polyadenylate polymerase, using ATP as the substrate. The pre-mRNA is cleaved downstream of a consensus sequence, called the polyadenylation signal sequence (AAUAAA), found near the 3′-end of the RNA, and the poly-A tail is added to the new 3′end. Tailing terminates eukaryotic transcription. Tails help stabilize the mRNA, facilitate its exit from the nucleus, and aid in translation. After the mRNA enters the cytosol, the poly-A tail is gradually shortened.
3.
Splicing: Maturation of eukaryotic mRNA usually involves removal from the primary transcript of RNA sequences (introns or intervening sequences) that do not code for protein. The remaining coding (expressed) sequences, the exons, are joined together to form the mature mRNA. The process of removing introns and joining exons is called splicing. The molecular complex that accomplishes these tasks is known as the spliceosome. A few eukaryotic primary transcripts contain no introns (for example, those from histone genes). Others contain a few introns, whereas some, such as the primary transcripts for the α chains of collagen, contain >50 introns that must be removed.
a. Role of small nuclear RNA: In association with multiple proteins, uracil-rich snRNA form five small nuclear ribonucleoprotein particles (snRNP, or “snurp”) designated as U1, U2, U4, U5, and U6 that mediate splicing. They facilitate the removal of introns by forming base pairs with the consensus sequences at each end of the intron (Fig. 31.18). [Note: In systemic lupus erythematosus, an autoimmune disease, patients produce antibodies against their own nuclear proteins such as snRNP.] b.