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You are watching: Is coupling of transcription and translation possible in single-celled eukaryotes such as yeast?

Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002.

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We turn now to warrior in eukaryotes, a lot more complicated process 보다 in prokaryotes. In eukaryotes, transcription and translation take ar in different cellular compartments: warrior takes place in the membrane-bounded nucleus, whereas translation take away place exterior the cell core in the cytoplasm. In prokaryotes, the two procedures are very closely coupled (Figure 28.15). Indeed, the translate in of bacter mRNA starts while the transcript is still being synthesized. The spatial and temporal separation that transcription and translation allows eukaryotes to regulate gene expression in much much more intricate ways, contributing come the wealth of eukaryotic type and function.


Figure 28.15

Transcription and also Translation. This two procedures are very closely coupled in prokaryotes, vice versa, they room spacially and also temporally separate in eukaryotes. (A) In prokaryotes, the main transcript serves together mRNA and also is used automatically as the theme (more...)

A second major difference in between prokaryotes and also eukaryotes is the degree of RNA processing. Back both prokaryotes and also eukaryotes modify tRNA and also rRNA, eukaryotes an extremely extensively process nascent RNA destined to come to be mRNA. Main transcripts (pre-mRNA molecules), the assets of RNA polymerase action, acquire a lid at their 5′ ends and also a poly(A) tail at your 3′ ends. Most importantly, almost all mRNA precursors in greater eukaryotes space spliced (Section 5.6.1). Introns are exactly excised from major transcripts, and also exons are joined to type mature mRNAs with constant messages. Part mRNAs are only a tenth the size of their precursors, which have the right to be as big as 30 kb or more. The pattern of splicing have the right to be regulated in the course of development to generate variations top top a theme, such as membrane-bound and also secreted creates of antibody molecules. Different splicing enlarges the repertoire of protein in eukaryotes and also is a clean illustration of why the proteome is more complicated than the genome.

28.2.1. RNA in eukaryotic Cells Is Synthesized by Three types of RNA Polymerase

In prokaryotes, RNA is synthesized by a single kind the polymerase. In contrast, the cell core of a eukaryote includes three species of RNA polymerase different in template specificity, ar in the nucleus, and also susceptibility to inhibitors (Table 28.2). Every these polymerases are big proteins, containing from 8 come 14 subunits and having a full molecular mass greater than 500 kd. RNA polymerase i is situated in nucleoli, wherein it transcribes the tandem range of genes for 18S, 5.8S, and also 28S ribosome RNA (Section 29.3.1). The various other ribosomal RNA molecule (5S rRNA, ar 29.3.1) and all the carry RNA molecule (Section 29.1.2) room synthesized through RNA polymerase III, i m sorry is situated in the nucleoplasm quite than in nucleoli. RNA polymerase II, which also is located in the nucleoplasm, synthesizes the precursors of messenger RNA as well as several tiny RNA molecules, such together those of the splicing apparatus (Section 28.3.5). Back all eukaryotic RNA polymerases are homologous come one another and also to prokaryotes RNA polymerase, RNA polymerase II consists of a distinct carboxyl-terminal domain on the 220-kd subunit; this domain is unusual due to the fact that it contains multiple repeats of a YSPTSPS consensus sequence. The tasks of RNA polymerase II room regulated through phosphorylation on the serine and also threonine residues of the carboxyl-terminal domain. One more major distinction among the polymerases lies in your responses to the toxin α-amanitin, a cyclic octapeptide that consists of several amendment amino acids.



α-Amanitin is produced by the poisonous mushroom Amanita phalloides, i m sorry is also called the death cup or the ruining angel (Figure 28.16). More than a hundred deaths result worldwide each year from the ingestion of poisonous mushrooms. α-Amanitin binds very tightly (Kd = 10 nM) come RNA polymerase II and also thereby blocks the elongation phase of RNA synthesis. Higher concentrations of α-amanitin (1 μM) inhibit polymerase III, whereas polymerase i is insensitive come this toxin. This pattern of sensitivity is highly conserved transparent the animal and also plant kingdoms.


Figure 28.16

RNA Polymerase Poison. Amanita phalloides, a poisonous mushroom the produces α-amanitin.

28.2.2. Cis- and Trans-Acting Elements: Locks and Keys that Transcription

Eukaryotic genes, like their prokaryotic counterparts, require promoters for warrior initiation. Every of the three species of polymerase has distinctive promoters. RNA polymerase i transcribes indigenous a single form of promoter, existing only in rRNA genes, that encompasses the initiation site. In some genes, RNA polymerase III responds come promoters located in the normal, upstream position; in various other genes, it responds to promoters imbedded in the genes, downstream the the initiation site. Promoters because that RNA polymerase II deserve to be simple or complex (Section 28.2.3). As is the case for prokaryotes, promoters are always on the exact same molecule that DNA as the gene they regulate. Consequently, promoters are referred to as cis-acting elements.

However, promoters are not the only varieties of cis-acting DNA sequences. Eukaryotes and their viruses additionally contain enhancers. These DNA sequences, although not promoters themselves, have the right to enormously boost the performance of promoters. Interestingly, the location of enhancers relative to promoters are not fixed; they have the right to vary substantially. Enhancers play an essential roles in regulation gene expression in a specific tissue or developmental phase (Section 31.2.4).

The DNA order of cis-acting elements are binding sites because that proteins called transcription factors. Together a protein is sometimes dubbed a trans-acting factor since it may be encoded by a gene ~ above a DNA molecule various other than that containing the gene gift regulated. The binding that a transcription element to that is cognate DNA sequence permits the RNA polymerase to find the ideal initiation site. Us will proceed our examination of warrior by examining these cis- and trans-acting aspects in turn.

28.2.3. Many Promoters because that RNA Polymerase II contain a TATA Box close to the Transcription begin Site

Promoters because that RNA polymerase II, like those for bacterial polymerases, are situated on the 5′ next of the start website for transcription. The outcomes of mutagenesis experiments, footprinting studies, and also comparisons that many greater eukaryotic genes have demonstrated the prestige of several upstream regions. For most genes transcribed by RNA polymerase II, the most vital cis-acting facet is called the TATA crate on the communication of its consensus sequence (Figure 28.17). The TATA box is usually centered in between positions -30 and -100. Note that the eukaryotic TATA box closely resembles the prokaryotic - 10 sequence (TATAAT) but is farther indigenous the start site. The mutation the a solitary base in the TATA crate markedly impairs promoter activity. Thus, the specific sequence, not just a high content of at pairs, is essential.

Figure 28.17

TATA Box. Comparisons of the order of an ext than 100 eukaryotic promoters resulted in the consensus sequence shown. The subscripts denote the frequency (%) the the base at that position.

The TATA crate is necessary but not sufficient for solid promoter activity. Additional elements space located in between -40 and also -150. Countless promoters contain a CAAT box, and also some contain a GC box (Figure 28.18). Constitutive genes (genes that are repeatedly expressed fairly than regulated) have tendency to have actually GC box in your promoters. The positions of this upstream sequences differ from one promoter to another, in contrast with the quite constant location that the -35 region in prokaryotes. One more difference is the the CAAT box and also the GC box deserve to be effective when existing on the theme (antisense) strand, uneven the -35 region, which must be present on the coding (sense) strand. This differences in between prokaryotes and eukaryotes reflect fundamentally various mechanisms for the acknowledgment of cis-acting elements. The -10 and also -35 sequences in prokaryotes promoters correspond to binding sites for RNA polymerase and its connected σ factor. In contrast, the TATA, CAAT, and also GC boxes and also other cis-acting elements in eukaryotic bio promoters are known by proteins various other than RNA polymerase itself.

Figure 28.18

CAAT Box and GC Box. Consensus sequences because that the CAAT and GC boxes of eukaryotic promoters for mRNA precursors.

28.2.4. The TATA-Box-Binding Protein Initiates the Assembly of the energetic Transcription Complex

Cis-acting aspects constitute only component of the puzzle of eukaryotic bio gene expression. Transcription determinants that tie to this elements also are required. Because that example, RNA polymerase II is guided come the start website by a set of transcription factors known collectively as TFII (TF means transcription factor, and II refers to RNA polymerase II). Separation, personal, instance TFII components are referred to as TFIIA, TFIIB, and also so on. Initiation begins with the binding that TFIID come the TATA box (Figure 28.19).

Figure 28.19

Transcription Initiation. Transcription components TFIIA, B, D, E, and also F are essential in initiating warrior by RNA polymerase II. The step-by-step assembly that these general transcription factors begins with the binding the TFIID (purple) come the TATA (more...)

The an essential initial event is the recognition of the TATA crate by the TATA-box-binding protein (TBP), a 30-kd ingredient of the 700-kd TFIID complex. TBP binding 105 times as tightly come the TATA box regarding noncognate sequences; the dissociation continuous of the specific facility is about 1 nM. TBP is a saddle-shaped protein consists of two comparable domains (Section 7.3.3; number 28.20). The TATA box of DNA binds to the concave surface ar of TBP. This binding induces huge conformational changes in the bound DNA. The double helix is substantially unwound to widen its young groove, permitting it to make extensive call with the antiparallel β strands on the concave side of TBP. Hydrophobic interaction are influential at this interface. 4 phenylalanine residues, for example, space intercalated in between base pairs of the TATA box. The flexibility of AT-rich assignment is typically exploited here in bending the DNA. Instantly outside the TATA box, classic B-DNA resumes. This facility is distinctly asymmetric. The asymmetry is crucial for specifying a distinctive start site and also ensuring that warrior proceeds unidirectionally.

Figure 28.20

Complex developed by TATA-Box-Binding Protein and DNA. The saddlelike structure of the protein sit atop a DNA fragment that is both significantly unwound and bent.

TBP bound to the TATA crate is the heart of the initiation complex (see number 28.19). The surface of the TBP saddle gives docking sites for the binding the other components (Figure 28.21). Extr transcription components assemble top top this cell core in a characterized sequence. TFIIA is recruited, followed by TFIIB and then TFIIF—an ATP-dependent helicase that initially separates the DNA duplex for the polymerase. Finally, RNA polymerase II and also then TFIIE join the other factors to type a complex called the basal transcription apparatus. At some time in the formation of this complex, the carboxyl-terminal domain the the polymerase is phosphorylated on the serine and also threonine residues, a process required for effective initiation. The prominence of the carboxyl-terminal domain is emphasize by the finding the yeast include mutant polymerase II with fewer 보다 10 repeats is not viable. Most of the components are released prior to the polymerase pipeline the promoter and can then get involved in an additional round of initiation.

Figure 28.21

Assembly of the Initiation Complex. A ternary complicated between the TATA-box-binding protein (purple), TFIIA (orange), and DNA. TFIIA interacts mostly with the other protein.

although bacteria absence TBP, archaea use a TBP molecule the is structurally quite similar to the eukaryotic protein. In fact, transcriptional control processes in archaea are, in general, much more similar to those in standard scale than room the procedures in bacteria. Many contents of the eukaryotic transcriptional machinery progressed from one ancestor that archaea.

28.2.5. Multiple warrior Factors communicate with eukaryotic Promoters

The basal transcription complex described in ar 28.2.4. Initiates warrior at a relatively low frequency. Additional transcription factors that tie to various other sites are required to attain a high rate of mRNA synthesis and also to selectively stimulate details genes. Upstream stimulatory sites in eukaryotic genes are diverse in sequence and variable in position. Their variety suggests the they are recognized by numerous different details proteins. Indeed, numerous transcription factors have been isolated, and also their binding sites have been established by footprinting experiments (Figure 28.22). Because that example, Sp1, one ~ 100-kd protein native mammalian cells, binding to promoters the contain GC boxes. The duplex DNA that SV40 virus (a cancer-producing virus that infects monkey cells) consists of five GC boxes indigenous 50 to 100 bp upstream or downstream of begin sites. The CCAAT-binding transcription variable (CTF; also called NF1), a 60-kd protein from mammalian cells, binds to the CAAT box. A heat-shock transcription aspect (HSTF) is express in Drosophila after ~ an abrupt rise in temperature. This 93-kd DNA-binding protein binding to the consensus sequence
Figure 28.22

Transcription-Factor-Binding Sites. This multiple binding sites because that transcription components were mapped through footprinting. (A) Binding the Sp1 (green) to the SV40 famous promoter and also to the dihydrofolate reductase (DHFR) promoter. (B) Binding the HSTF (blue) (more...)

Several duplicates of this sequence, well-known as the heat-shock an answer element, room present beginning at a website 15 bp upstream of the TATA box. HSTF different from σ32, a heat-shock protein of E. Coli (Section 28.1.2), in binding straight to response elements in heat-shock promoters fairly than very first becoming associated with RNA polymerase.

28.2.6. Enhancer Sequences have the right to Stimulate transcription at begin Sites hundreds of Bases Away

The activities of countless promoters in higher eukaryotes are considerably increased by another type of cis-acting aspect called an enhancer. Enhancers" sequences have actually no promoter activity of their own yet deserve to exert their stimulatory plot over ranges of numerous thousand base pairs. They deserve to be upstream, downstream, or also in the middle of a transcribed gene. Moreover, enhancers are effective when present on either DNA strand (equivalently, in one of two people orientation). Enhancers in yeast are recognized as upstream activator order (UASs).

A specific enhancer is efficient only in particular cells. Because that example, the immunoglobulin enhancer features in B lymphocytes yet not elsewhere. Cancer can an outcome if the relation between genes and enhancers is disrupted. In Burkitt lymphoma and also B-cell leukemia, a chromosomal translocation brings the proto-oncogene myc (a transcription variable itself) under the control of a an effective immunoglobin enhancer. The consequent dysregulation the the myc gene is thought to beat a role in the progression of the cancer.

Transcription factors and other proteins that tie to regulatory sites ~ above DNA have the right to be concerned as passwords that cooperatively open up multiple locks, giving RNA polymerase accessibility to specific genes. The exploration of promoters and also enhancers has opened the door to understanding how genes room selectively to express in eukaryotic bio cells. The regulation of gene transcription, discussed in chapter 31, is the basic means of managing gene expression.

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By agreement with the publisher, this book is easily accessible by the search feature, however cannot it is in browsed.