As introduced in the description of tRNAs, essentially all are covalently modified after transcription. It is clear that at least some of these modifications are regulatory.

Messenger RNA Is Extensively Modified Both Internally & at 5′ & 3′Termini

Eukaryotic mRNAs contain a 7-methylguanosine cap structure at their 5′ terminus, and most have a poly(A) tail at the 3′ terminus. The cap structure is added to the 5′ end of the newly transcribed mRNA precursor in the nucleus very soon after synthesis and prior to transport of the mRNA molecule to the cytoplasm. The 5′ cap of the RNA transcript is required both for efficient translation initiation and protection of the 5′ end of mRNA from attack by 5′ → 3′ exonucleases. The secondary methylations of mRNA molecules, those on the 2′-hydroxy and the N7 of adenylyl residues, occur after the mRNA molecule has appeared in the cytoplasm.

Poly(A) tails are added to the 3′ end of mRNA molecules in a posttranscriptional processing step. The mRNA is first cleaved about 20 nucleotides downstream from an AAUAA recognition sequence. Another enzyme, poly(A) polymerase, adds a poly(A) tail which is subsequently extended to as many as 200 A residues. The poly(A) tail both protects the 3′ end of mRNA from 3′ → 5′ exonuclease attack and facilitates translation. The presence or absence of the poly(A) tail does not determine whether a precursor molecule in the nucleus appears in the cytoplasm, because all poly(A)-tailed nuclear mRNA molecules do not contribute to cytoplasmic mRNA, nor do all cytoplasmic mRNA molecules contain poly(A) tails (histone mRNAs are most notable in this regard). Following nuclear transport, cytoplasmic enzymes in mammalian cells can both add and remove adenylyl residues from the poly(A) tails; this process has been associated with an alteration of mRNA stability and translatability.

The size of some cytoplasmic mRNA molecules, even after the poly(A) tail is removed, is still considerably greater than the size required to code for the specific protein for which it is a template, often by a factor of 2 or 3. The extra nucleotides occur in untranslated (nonprotein coding) exonic regions both 5′ and 3′ of the coding region; the longest untranslated sequences are usually at the 3′ end. The 5′ UTR and 3′ UTR sequences have been implicated in RNA processing, transport, storage, degradation, and translation; each of these reactions potentially contributes additional levels of control of gene expression. Some of these posttranscriptional events involving mRNAs occur in cytoplasmic organelles termed P bodies.

Recent research has shown that eukaryotic mRNAs are subject to extensive and dynamic posttranscriptional modifications. The best characterized of these base modifications is N6-methyladenosine, in which the amino group attached to the C6 position of adenine is methylated. As described for histone-specific posttranslational modifications and functions and regulatory phosphorylation of protein/function-activity, distinct families of enzymes/proteins that attach (“writers”), and bind (“readers”), and remove (“erasers”) the methyl group of N6-methyl adenosine have been described and characterized. Genetic deletion experiments wherein the genes encoding these writers/readers/erasers are mutated all show that the N6-methyladenosine mRNA “mark” contributes importantly to cell viability and function. Future work will uncover the extent to which mRNA posttranscriptional modifications support human cell function and impact disease.

Micro-RNAs Are Derived From Large Primary Transcripts Through Specific Nucleolytic Processing

 The majority of miRNAs are transcribed by RNA pol II into primary transcripts termed pri-miRNAs. pri-miRNAs are 5′-capped and 3′-polyadenylated (Figure 1). pri-miRNAs are synthesized from transcription units encoding one or several distinct miRNAs; these transcription units are either located independently in the genome, or within the intronic DNA of other genes. Given this organization miRNA-encoding genes must therefore minimally possess a distinct promoter, coding region, and polyadenylation/termination signals. pri-miRNAs have extensive 2° structure, and this intramolecular structure is maintained following processing by the Drosha-DGCR8 nuclease; the portion containing the RNA hairpin is preserved, transported through the nuclear pore via the action of exportin 5, and once in the cytoplasm, further processed by the het erodimeric dicer nuclease-TRBP complex to a 21 or 22-mer. Ultimately, one of the two strands is selected for loading into the RNA-induced silencing complex (RISC), which is com posed of one of four Argonaute proteins (Ago 1→4), to form a mature, functional 21–22 nt single-stranded miRNA. siRNAs are produced similarly. Once in the RISC complex, miRNAs can modulate mRNA function by one of three mechanisms: (a) promoting mRNA degradation directly; (b) stimulating CCR4/NOT complex-mediated poly(A) tail degradation; or (c) inhibition of translation by targeting the 5′-methyl cap binding translation factor eIF4 or the ribosome directly. Recent data suggest that at least some regulatory miRNA-encoding genes may be linked, and hence coevolve with their target genes.

Fig1. Biogenesis of micro (mi) and silencing (si)RNAs. (Left) miRNA encoding genes are transcribed by RNA pol II into a primary miRNA (pri-miRNA), which is 5′-capped and polyadenylated as is typical of mRNA coding primary transcripts. This pri-miRNA is subjected to processing within the nucleus by the action of the Drosha-DGCR8 nuclease, which trims sequences from both 5′ and 3′ ends to generate the pre-miRNA. This partially processed double-stranded RNA is transported through the nuclear pore by exportin-5. The cytoplasmic pre-miRNA is then trimmed further by the action of the heterodimeric nuclease termed Dicer (TRBP-Dicer), to form the 21–22 nt miRNA duplex. One of the two resulting 21–22 nucleotide-long RNA strands is selected, the duplex unwound, and the selected strand loaded into the RNA-induced silencing complex, or RISC complex containing one of several Ago proteins (Ago1, 2, 3, or 4) and other important accessory proteins, thereby generating the mature, functional miRNA. Following target mRNA location and sequence-specific miRNA–mRNA annealing, the functional miRNA can modulate mRNA function by one of three mechanisms: translational repression, mRNA destabilization by mRNA deadenylation, or mRNA degradation. (Right) The siRNA pathway generates functional siRNAs from large double-stranded RNAs that are formed either intracellularly by RNA–RNA hybridization (inter- or intramolecular) or from extracellular sources such as RNA viruses. These viral dsRNAs are again processed to ~22 nt siRNA dsRNA segments via the heterodimeric Dicer nuclease, loaded into the Ago2-containing RISC complex, one strand is then selected to generate the siRNA that locates target RNA sequences via sequence-specific siRNA–RNA annealing. This ternary target RNA–siRNA–Ago2 complex induces RNA cleavage, which inactivates the target RNA.

RNA Editing Alters mRNA Sequence After Transcription

The central dogma states that for a given gene and gene product there is a linear relationship between the coding sequence in DNA, the mRNA sequence, and the protein sequence. Changes in the DNA sequence should be reflected in a change in the mRNA sequence and, depending on codon usage, in protein sequence. However, exceptions to this dogma have been documented. Coding information can be changed at the mRNA level by RNA editing. In such cases, the coding sequence of the mRNA differs from that in the cognate DNA. An example is the apolipoprotein B (apoB) gene and mRNA. In liver, the single apoB gene is transcribed into an mRNA that directs the synthesis of a 100-kDa protein, apoB100. In the intestine, the same gene directs the syn thesis of the identical mRNA primary transcript; however, a cytidine deaminase converts a CAA codon in the mRNA to UAA at a single specific site. Rather than encoding glutamine, this codon becomes a termination signal, and hence production of a truncated 48-kDa protein (apoB48) results. ApoB100 and apoB48 have different functions in the two organs. A growing number of other examples include a glutamine to arginine change in the glutamate receptor and several changes in trypanosome mitochondrial mRNAs, generally involving the addition or deletion of uridine. Current estimates suggest that perhaps 0.01% of mRNAs are edited in this fashion. Recently, editing of miRNAs has been described suggesting that these two forms of posttranscriptional control mechanisms could cooperatively contribute to gene regulation. Enzymes that catalyze RNA editing have been identified and characterized extensively. Scientists have been able to harness these enzymes to “correct” mutated mRNAs derived from disease-causing gene variants using cell culture models. These exciting new results offer yet another molecular genetic technology to potentially mitigate, or cure human disease.

Transfer RNA Is Extensively Processed & Modified

 As described in Chapter 34 and detailed in Chapter 37, tRNA molecules serve as adapter molecules for the translation of mRNA into protein sequences. tRNAs contain many modifications of the standard bases A, U, G, and C, including methylation, reduction, deamination, and rearranged glycosidic bonds. Further posttranscriptional modification of the tRNA molecules includes nucleotide alkylations and the attachment of the characteristic CpCpAOH terminal at the 3′ end of the molecule by a nucleotidyl transferase. The 3′ OH of the A ribose is the point of attachment for the specific amino acid that is to enter the polymerization reaction of protein synthesis. The methylation of mammalian tRNA precursors probably occurs in the nucleus, whereas the cleavage and attachment of CpCpAOH are cytoplasmic functions; the terminal nucleotides turn over more rapidly than do the tRNA molecules themselves. Specific amino acyl tRNA synthetases within the cytoplasm of mammalian cells are required for the attachment of the different amino acids to the CpCpAOH residues.

اخر الاخبار

اخبار العتبة العباسية المقدسة

لزيادة المساحات الخضراء.. قسم الحزام الأخضر الثاني يشرع بحملة تشجير في جامعة العميد

اللجنة التحضيرية لمهرجان روح النبوة تعلن تمديد موعد تسلم البحوث الخاصة بالمسابقة البحثية

المجمع العلمي يطلق جلستين قرآنيتين لطلبة العلوم الدينية في النجف

قسم الشؤون الفكرية يقدم محاضرة حول أساليب الحوار الإيجابي لوفد شبابي من البصرة

المزيد

الآخبار الصحية

هل ممارسة الرياضة على معدة فارغة تحرق الدهون أكثر؟

العلم يوضح الوقت "الأمثل" لشرب القهوة

دراسة: "طريقة سهلة" لحماية رئتيك من آثار تلوث الهواء

بالذكاء الاصطناعي.. أداة جديدة لتحسين علاج الصرع عند الأطفال

المزيد

الآخبار التكنلوجية

إيلون ماسك يعتزم إطلاق "غروكيبيديا" كمنافس لـ "ويكيبيديا"

"قرار" من أبل لحماية هواتف آيفون 17 من الخدوش

ميزة جديدة على "تشات جي بي تي".. شراء المنتجات من المحادثة

الصين تكشف عن توربين رياح طائر يغير قواعد الطاقة المتجددة

المزيد

مواضيع ذات صلة

Posttranslational Processing Affects the Activity of many Proteins

Like Transcription, Protein synthesis can be described in three phases: Initiation, Elongation, & Termination

Mutations result when changes occur in the Nucleotide Sequence

The Genetic Code is Degenerate, Unambiguous, Nonoverlapping, Without Punctuation, & Universal

RNA Molecules are Extensively Processed Before They Become Functional

Phosphorylation Activates Pol II

Formation of the Pol II Transcription Complex

Eukaryotic Promoters Are More Complex

Bacterial Promoters Are Relatively Simple

RNA Synthesis is A Cyclical Process that Involves RNA Chain Initiation, Elongation, & Termination

RNA is Synthesized From A DNA Template by RNA Polymerases

DNA & Chromosome Integrity Is Monitored Throughout the Cell Cycle