transcription eukar

7/30/2019 Transcription Eukar

1/3

Eukaryotic TranscriptionEukaryotic transcription uses three distinct RNA polymerases, which are specialized

for different RNAs. RNA polymerase I makes Ribosomal RNAs, RNA polymerase

II makes messenger RNAs, and RNA polymerase III makes small, stable RNAs

such as transfer RNAs and 5S ribosomal RNA. Eukaryotic RNA polymerases are

differentiated by their sensitivity to the toxic compound, -amanitin, the active

compound in the poisonous mushroomAminita phalloides,or destroying angel.

RNA polymerase I is not inhibited by -amanitin, RNA polymerase II is inhibited at

very low concentrations of the drug, and RNA polymerase III is inhibited at high

drug concentrations.

Eukaryotic transcription is dependent on several sequence and structural features.

First, actively transcribing genes have a looser, more accessible chromatin

structure. The nucleosomes are not as condensed as in other forms of chromatin,

especially heterochromatin, and they often do not contain histone H1. The DNA inthe promoter region at the 5 end of the gene may not be bound into nucleosomes

at all. In this way, the promoter sequences are available for binding to

proteintranscription factorsproteins that bind to DNA and either repress or

stimulate transcription. In addition to promoter sequences, other nucleotide

sequences termed enhancers can affect transcription efficiency. Enhancers bind to

specialized protein factors and then stimulate transcription. The difference between

enhancers and factor-binding promoters depends on their site of action. Unlike

promoters, which only affect sequences immediately adjacent to them, enhancers

function even when they are located far away (as much as 1,000 base pairs away)

from the promoter. Both enhancer-binding and promoter-binding transcription

factors recognize their appropriate DNA sequences and then bind to other proteins

for example, RNA polymerase, to help initiate transcription. Because enhancers are

located so far from the promoters where RNA polymerase binds, enhancer

interactions involve bending the DNA to make a loop so the proteins can interact.

Ribosomal RNA synthesis

Most of the RNA made in the cell is ribosomal RNA. The large and small subunit

RNAs are synthesized by RNA polymerase I. Ribosomal RNA is made in a specialized

organelle, the nucleolus, which contains many copies of the rRNA genes, acorrespondingly large number of RNA polymerase I molecules, and the cellular

machinery that processes the primary transcripts into mature rRNAs. RNA

polymerase I is the most abundant RNA polymerase in the cell, and it synthesizes

RNA at the fastest rate of any of the polymerases. The genes for rRNA are present in

many copies, arranged in tandem, one after the other. Each transcript contains a

copy of each of three rRNAs: the 28S and 5.8S large subunit RNAs and the small

subunit 18S RNA, in that order. The rRNA promoter sequences extend much further

upstream than do prokaryotic promoters. The transcription of rRNA is very efficient.

This is necessary because each rRNA transcript can only make one ribosome, in

contrast to the large number of proteins that can be made from a single mRNA.


2/3

The individual ribosomal RNAs must be processed from the large precursor RNA that

is the product of transcription. The primary transcript contains small and large

subunit RNAs in the order: 28S5.8S18S. Processing involves the modification of

specific nucleotides in the rRNA, followed by cleavage of the transcript into the

individual RNA components. See Figure1.

Figure 1Messenger RNA transcription

RNA polymerase II transcribes messenger RNA and a few other small cellular RNAs.

Class II promoters are usually defined by their sensitivity to -amanitin. Like

prokaryotic promoters, many class II promoters contain two conserved sequences,

called the CAAT and TATA boxes. The TATA box is bound by a specialized

transcription factor called TBP (for TATA-Binding-Factor). Binding of TBP is required

for transcription, but other proteins are required to bind to the upstream (and

potentially downstream) sequences that are specific to each gene. Like prokaryotic

transcripts, eukaryotic RNAs are initiated with a nucleoside triphosphate.

Termination of eukaryotic mRNA transcription is less well understood than is

termination of prokaryotic transcription, because the 3 ends of eukaryotic mRNAs

are derived by processing. See Figure2.

Figure 2Transfer and 5S ribosomal RNA transcription

RNA polymerase III transcribes 5S rRNA and tRNA genes. The promoter of these

transcripts can actually be located inside the gene itself, in contrast to all the other

promoters discussed earlier. See Figure3.
http://www.cliffsnotes.com/study_guide/Eukaryotic-Transcription.topicArticleId-24594,articleId-24589.html#schmidt5622c12-fig-0008http://www.cliffsnotes.com/study_guide/Eukaryotic-Transcription.topicArticleId-24594,articleId-24589.html#schmidt5622c12-fig-0008http://www.cliffsnotes.com/study_guide/Eukaryotic-Transcription.topicArticleId-24594,articleId-24589.html#schmidt5622c12-fig-0008http://www.cliffsnotes.com/study_guide/Eukaryotic-Transcription.topicArticleId-24594,articleId-24589.html#schmidt5622c12-fig-0009http://www.cliffsnotes.com/study_guide/Eukaryotic-Transcription.topicArticleId-24594,articleId-24589.html#schmidt5622c12-fig-0009http://www.cliffsnotes.com/study_guide/Eukaryotic-Transcription.topicArticleId-24594,articleId-24589.html#schmidt5622c12-fig-0009http://www.cliffsnotes.com/study_guide/Eukaryotic-Transcription.topicArticleId-24594,articleId-24589.html#schmidt5622c12-fig-0010http://www.cliffsnotes.com/study_guide/Eukaryotic-Transcription.topicArticleId-24594,articleId-24589.html#schmidt5622c12-fig-0010http://www.cliffsnotes.com/study_guide/Eukaryotic-Transcription.topicArticleId-24594,articleId-24589.html#schmidt5622c12-fig-0010http://www.cliffsnotes.com/study_guide/Eukaryotic-Transcription.topicArticleId-24594,articleId-24589.html#schmidt5622c12-fig-0010http://www.cliffsnotes.com/study_guide/Eukaryotic-Transcription.topicArticleId-24594,articleId-24589.html#schmidt5622c12-fig-0009http://www.cliffsnotes.com/study_guide/Eukaryotic-Transcription.topicArticleId-24594,articleId-24589.html#schmidt5622c12-fig-0008


3/3

Figure 3The 5 sequence is not essential for accurate transcription initiation. When the region

extending from the 5 end of the gene (that is, the part that would normally be

considered to be the promoter) is deleted, RNA synthesis is carried out just as

efficiently as on the native gene. The new 5 end of the transcript is complementaryto whatever sequences take the place of the natural ones. Furthermore, initiation is

only affected when sequences within the 5S rRNA gene are disrupted. The molecular

explanation for this phenomenon is as follows:

1. A protein factor binds to the 5S rRNA gene. Binding is at the internal sequencethat is required for accurate initiation.

2. The bound factor then interacts with RNA polymerase III, which is thencapable of initiation. During transcription, the multiple protein factors (called

TFIIIs) remain bound to the transcribing gene.

transcription eukar

Documents