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118 Metabolism
Transcription control B. Lactose operon
The well-investigated lactose operon of the
A. Functioning of regulatory proteins bacterium Escherichial coli can be used here
as an example of transcriptional control. The
Regulatory proteins (transcription factors) are
involved in controlling gene expression in all lac operon is a DNA sequence that is simul-
taneously subject to negative and positive
cells. These regulatory proteins bind to spe- control. The operon contains the structural
cific DNA sequences and thereby activate or
genes for three proteins that are required for
inhibit the transcription of genes (Tran- the utilization of lactose (one transporter and
scription control). The effects of transcription
factors are usually reversible and are often two enzymes), as well as control elements that
serveto regulatethe operon.
controlled by ligands or by interconversion. Since lactose is converted to glucose in the
The nomenclature for transcription factors cell, there is no point in expressing the genes
is confusing. Depending on their mode of ac- if glucose is already available. And indeed, the
tion, various terms are in use both for the
proteins themselves and for the DNA sequen- genes are in fact only transcribed when glu-
cose is absent and lactose is present (3). This is
ces to which they bind. If a factor blocks tran-
scription, it is referred to as a repressor;oth- achieved by interaction between two regula-
erwise, it is called an inducer. DNA sequences tory proteins. In the absence of lactose, the lac
repressor blocks the promoter region (2).
to which regulatory proteins bind are referred
to as control elements. In prokaryotes, control When lactose is available, it is converted
into allolactose, which binds to the repressor
elements that serve as binding sites for RNA and thereby detaches it from the operator (3).
polymerases are called promoters, whereas However, this is still not suf cient for the
repressor-binding sequences are usually transcription of the structural genes. For bind-
called operators. Control elements that bind
activating factors are termed enhancers, ingofthe RNApolymeraseto takeplace,an
inducer—the
while elements that bind inhibiting factors catabolite activator protein
are known as silencers. (CAP)—is required, which only binds to the
DNA when it is present as a complex with
The numerous regulatory proteins that are
known can be classified into four different 3,5 -cyclo-AMP (cAMP; see p. 386). cAMP, a
groups (1–4), based on their mechanisms of signal for nutrient deficiency, is only formed
by E. coli in the absence of glucose.
action. Negative gene regulation—i. e., The interaction between the CAP–cAMP
switching off of the gene concerned—is car- complex and DNA is shown in Fig. 4.Each
ried out by repressors. Some repressors only subunit of the dimeric inducer (yellow or or-
bind to DNA (1a) in the absence of specific
ligands (L). In this case, the complex between ange) binds one molecule of cAMP (red). Con-
tact with the DNA (blue) is mediated by two
the repressor and the ligand loses its ability to
bind to the DNA, and the promoter region “recognition helices” that interact with the
majorgrooveofthe DNA. Thebending of the
becomes accesible for binding of RNA poly- DNA strandcausedbyCAP hasfunctional sig-
merase (1b). It is often the free repressor that
does not bind to the DNA, so that transcrip- nificance.
Transcription control is much more com-
tion is only blocked in the presence of the
ligand (2a, 2b). A distinction between two plex in eukaryotes (see p. 244). The number
of transcription factors involved is larger, and
different types of positive gene regulation in addition the gene activity is influenced by
canbe made inthe same way. If it is only
the free inducer that binds, then transcription the state of the chromatin (see p. 238).
is inhibited by the appropriate ligand (3).
Conversely, many inducers only become ac-
tive when they have bound a ligand (4). This
group includes the receptors for steroid hor-
mones, for example (see p. 378).
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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