Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

CHAPTER 8 END-OF-CHAPTER PROBLEMS
527
(A)
ACTIVATING
INPUT
GENE X
X
(B)
ACTIVATING
INPUT
GENE X
X
concentration of Y
33
1
2
GENE Y
GENE Y
Y
Y
concentration of X
GENE Z
Z
GENE Z
Z
Figure Q8–6 Perturbations of a bistable system (Problem 8–18). As
shown by the green lines, after perturbation 1 the system returns to
its original stable state (green dot at left), and after perturbation 2, the
system moves to the other stable state (green dot at right). Perturbation
3 moves the system to the precise boundary between the two stable
states (orange dot).
(C)
ACTIVATING
INPUT
GENE X
X
GENE Y
Y
(D)
GENE Z
ACTIVATING
INPUT
GENE X
X
GENE Y
Y
GENE Z
A. Which single operator site is the most important
for repression? How can you tell?
B. Do combinations of operator sites (Figure Q8–7,
constructs 1, 2, 3, and 5) substantially increase repression
by the dimeric repressor? Do combinations of operator
sites substantially increase repression by the tetrameric
repressor? If the two repressors behave differently, offer an
explanation for the difference.
C. The wild-type repressor binds O 3 very weakly
when it is by itself on a segment of DNA. However, if O 1 is
included on the same segment of DNA, the repressor binds
O 3 quite well. How can that be?
Z
Z
92 bp 401 bp 2-mer 4-mer
Figure Q8–5 Network motifs composed of transcription activators and
repressors (Problem 8–17).
8–19 Detailed analysis of the regulatory region of the
Lac operon has revealed surprising complexity. Instead
of a single binding site for the Lac repressor, as might be
expected, there are three sites termed operators: Figure 8-403 O 1 , O 2 ,
and O 3 , arrayed along the DNA as shown in Figure Q8–7.
To probe the functions of these three sites, Problem you make 8-310 a
series of constructs in which various combinations of operator
sites are present. You examine their ability to repress
expression of β-galactosidase, using either tetrameric
(wild type) or dimeric (mutant) forms of the Lac repressor.
The dimeric form of the repressor can bind to a single
operator (with the same affinity as the tetramer) with each
monomer binding to half the site. The tetramer, the form
normally expressed in cells, can bind to two sites simultaneously.
When you measure repression of β-galactosidase
expression, you find the results shown in Figure Q8–7, with
higher numbers indicating more effective repression.
1
2
3
4
5
6
7
8
O 3
O 1
O 2
O 3
O 1
Figure 8-404
Problem 8-311
O 1
O 2
O 1
O 3
O 2
O 3
O 2
Figure Q8–7 Repression of β-galactosidase by promoter regions that
contain different combinations of Lac repressor binding sites (Problem
8–19). The base-pair (bp) separation of the three operator sites is
shown. Numbers at right refer to the level of repression, with higher
numbers indicating more effective repression by dimeric (2-mer) or
tetrameric (4-mer) repressors. (From S. Oehler et al., EMBO J.
9:973–979, 1990. With permission from John Wiley and Sons.)
110
90
80
60
1
1
1
1
6700
3900
1400
140
5
2
1
1