BIOCHEMISTRY - L. Stryer - 1984

VOLUME 3

Part IV INFORMATION

CHAPTER 29. EUKARYOTIC CHROMOSOMES AND GENE EXPRESSION IN EUKARYOTES

29.20. Three types of ribosomal RNA are generated by the processing of a single primary transcript

RNA molecules synthesized by the three RNA polymerases are referred to as primary transcripts. These newly formed RNAs undergo extensive modification in The Nucleus before being exported to the Cytosol as mature rRNA, tRNA, and mRNA molecules. The formation of Ribosomal RNAs from a primary transcript has been studied in greater detail than the Processing of any other type of RNA. As discussed above, the genes for 18S, 5.8S, and 28S rRNAs are grouped into a single cluster, and this Gene cluster is repeated in tandem many times. In the nucleoli, this three-gene set is transcribed by RNA polymerase I to yield a 45S RNA (Fig. 29.30).

Class="center">Fig. 29.30. When a 45S ribosomal RNA precursor molecule from HeLa Cells is spread out and examined under an Electron microscope, a highly characteristic pattern of hairpins and loops is revealed. This allows mapping of the locations of the 28S and 18S RNA molecules derived from this precursor. A — electron micrograph of the 45S precursor. B — schematic drawing of the molecule shown in Fig. A

The 13 kb precursor undergoes enzymatic modification and Cleavage to yield mature 18S, 5.8S, and 28S rRNAs.

(Fig. 29.31). Approximately 100 NUCLEOTIDES are methylated, almost all of them at the 2'-hydroxyl group of the ribose residues. Highly specific methylation sites are conserved in rRNAs of evolutionarily distant eukaryotes such as Yeast and the fruit fly. In addition, more than 100 uridine residues are isomerized into pseudouridine residues. During maturation, numerous ribosomal Proteins become associated with these RNAs and their precursors. It is likely that this interaction between RNA and ribosomal proteins renders specific regions susceptible to nuclease action.

Fig. 29.31. Formation of mammalian ribosomal RNA from a primary transcript. Spacer regions are shown in yellow

29.21. Messenger RNAs are selectively formed from large nuclear RNA precursors (heterogeneous nuclear RNA, hnRNA)

The formation of ribosomal RNA in eukaryotes is similar to that in prokaryotes (Section 25.17). At the same time, there are substantial differences between eukaryotic and prokaryotic mRNAs.

1. Eukaryotic primary transcripts are not used directly as mRNA. Before being exported from the nucleus to the cytosol, they undergo processing. Translation and METABOLISM/31.html">Transcription in eukaryotes are uncoupled in time and space, whereas in prokaryotes they are tightly coupled.

2. Eukaryotic primary transcripts range from 2 to 20 kb in length, which is why they are referred to as heterogeneous nuclear RNA (hnRNA). Typically, these primary transcripts are several times longer than the mRNAs derived from them. The formation of eukaryotic mRNA involves splicing and cleavage. It remains unknown whether hnRNA serves any function other than acting as an mRNA precursor.

3. Eukaryotic mRNAs contain "caps" at their 5' ends, which consist of modified nucleotides. In addition, most mRNAs carry a long poly(A) tail at their 3' end.

4. Eukaryotic mRNAs are monocistronic, meaning they serve as templates for the synthesis of only a single polypeptide chain. Many prokaryotic mRNAs, by contrast, are polycistronic (for example, the mRNA of the lactose Operon serves as a template for the synthesis of three polypeptide chains).

5. The population of mRNA molecules in a Introduction/5.html">Eukaryotic Cell depends not only on the transcription rate of specific genes; Eukaryotic cells possess an additional regulatory checkpoint: whether a given primary transcript should undergo degradation or be processed to form a mature mRNA followed by its transport into the cytosol¹.

¹ Another important distinction between prokaryotic and eukaryotic mRNAs is their lifespan. In prokaryotes, it is no more than a few minutes, whereas in eukaryotes it ranges from tens of minutes to hours, and in exceptional cases, weeks or months. — Trans. note.

29.22. mRNA molecules feature caps at their 5' ends and, as a rule, poly(A) tails at their 3' ends

The 5' end of all known eukaryotic mRNAs (but not tRNAs or rRNAs) is modified in a distinct manner. A 7-methylguanylate is attached to the mRNA via an unusual 5'—5' pyrophosphate linkage (Fig. 29.32). This characteristic Structure is called a "cap." It is added to the primary transcript: the 5'-triphosphate end of the newly formed chain is hydrolyzed to a diphosphate, and a GTP guanylate residue is transferred to it. Subsequently, the N-7 atom of this terminal guanine is methylated by S-adenosylmethionine to form the so-called cap 0. Neighboring ribose residues may also be methylated, yielding cap 1 or cap 2 (Fig. 29.32). These caps contribute to mRNA stabilization by protecting their 5' ends from Phosphatases and Nucleases. Furthermore, caps enhance the translational efficiency of mRNA in eukaryotic protein-synthesizing systems.

¹ Recent evidence suggests the involvement of Small nuclear RNAs (snRNAs) in splicing and in the maturation of 3' ends of RNA. This RNA fraction has been studied for quite some time; it is highly conserved and contains sequences complementary to exon regions adjacent to cistrons. Small nuclear RNAs apparently facilitate the recognition of splice sites by the respective Enzymes. — Trans. note.

Fig. 29.32. STRUCTURE OF THE caps located at the 5' end of eukaryotic mRNAs. All caps contain 7-methylguanylate (shown in blue) attached via a pyrophosphate linkage to the 5' end. In cap 0, neither ribose is methylated; in cap 1, one ribose is methylated; in cap 2, two are methylated

In addition, most eukaryotic mRNAs feature a poly(A) tail at the 3' end. Poly(A) polymerase adds 150–200 nucleotides to the primary transcript, which contains a GC dinucleotide at the 3' end and an AAUAA sequence located approximately 20 nucleotides upstream. Studies of various mRNAs injected into Xenopus oocytes have shown that the poly(A) segment increases mRNA stability, yet plays no essential role in translation. Furthermore, the absence of a poly(A) tail on histone mRNAs demonstrates that it is not strictly required for mRNA Transport from the nucleus to the cytosol.