Page 251 - Color Atlas of Biochemistry

242 Molecular genetics
Transcription
For the genetic information stored in DNA to
become effective, it has to be rewritten
(transcribed) into RNA. DNA only serves as a
template and is not altered in any way by the
transcription process. Transcribable segments
of DNA that code for a defined product are
called genes. It is estimated that the mamma-
lian genome contains 30 000–40 000 genes,
which together account for less than 5% of the
DNA.
A. Transcription and maturation of RNA:
overview
Transcription is catalyzed by DNA–dependent
RNA polymerases. These act in a similar way to
DNA polymerases (see p. 240), except that
they incorporate ribonucleotides instead of
deoxyribonucleotides into the newly synthe-
sized strand; also, they do not require a pri-
mer. Eukaryotic cells contain at least three
different types of RNA polymerase. RNA poly-
merase I synthesizes an RNA with a sedimen-
tation coef cient (see p. 200) of 45 S, which
serves as precursor for three ribosomal RNAs.
The products of RNA polymerase II are
hnRNAs, from which mRNAs later develop,
as well as precursors for snRNAs. Finally,
RNA polymerase III transcribes genes that
code for tRNAs, 5S rRNA, and certain snRNAs.
These precursors giveriseto functionalRNA
molecules by a process called RNA maturation
(see p. 246). Polymerases II and III are inhib- is repeatedly phosphorylated, frees itself from
ited by D–amanitin, a toxin in the Amanita
phalloides mushroom.
B. Organization of the PEP-CK gene
The way in which a typical eukaryotic gene is
organized is illustrated here using a gene that step by step to the hnRNA as it grows in the
codes for a key enzyme in gluconeogenesis
(see p. 154)—the phosphoenolpyruvate car-
boxykinase (PEP-CK).
In the rat, the PEP-CK gene is nearly 7 kbp
(kilobase pairs) long. Only 1863 bp, distrib-
uted over 10 coding segments (exons,dark
blue) carry the information for the protein’s
621 amino acids. The remainder is allotted to
the promoter (pink) and intervening sequen-
ces (introns,light blue). Thegene’s promoter
region (approximately 1 kbp) serves for reg-
ulation (see p. 188). Transcription starts at the
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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3 end of the promoter (“transcription start”)
and continues until the polyadenylation se-
quence (see below) is reached. The primary
transcript (hnRNA) still has a length of about
6.2 kbp. During RNA maturation, the non-
coding sequences corresponding to the in-
trons are removed, and the two ends of the
hnRNA are modified. The translatable mRNA
still has half the length of the hnRNA and is
modified at both ends (see p. 246).
In many eukaryotic genes, the proportion
of introns is even higher. For example, the
gene for dihydrofolate reductase (see p. 402)
is over 30 kbp long. The information is dis-
tributed over six exons, which together have a
length of only about 6 kbp.
C. Transcription process
As mentioned above, RNA polymerase II
(green) binds to the 3 end of the promoter
region. A sequence that is important for this
binding is known as the TATA box—a short A–
and T–rich sequence that varies slightly from
gene to gene. A typical base sequence (“con-
sensus sequence”) is ...TATAAA... Numerous
proteins known as basal transcription factors
are necessary for the interaction of the poly-
merase with this region. Additional factors
can promote or inhibit the process (transcrip-
tional control; see p. 244). Together with the
polymerase, they form the basal transcription
complex.
At the end of initiation (2), the polymerase
the basal complex, and starts moving along
the DNA in the 3 direction. The enzyme sep-
arates a short stretch of the DNA double helix
into two single strands. The complementary
nucleoside triphosphates are bound by base
pairing in the template strand and are linked
5 3 direction (3). Shortly after the begin-
ning of elongation,the 5 end of the transcript
is protected by a “cap” (see p. 246). Once the
polyadenylation sequence has been reached
(typical sequence: ...AATAA...), the transcript
is released (4). Shortly after this, the RNA
polymerase stops transcribing and dissociates
from the DNA.