596
chapter 26
Regulation of Gene Expression
that binds to the operator. When the external supply of tryp-
tophan is depleted (or reduced substantially), the operator
becomes exposed, and transcription begins. This type of
on-off mechanism—activation of an aporepressor by the
product of the biosynthetic pathway—has been observed
in other biosynthetic pathways.
When the trp operon is derepressed, which is usually the
case unless the concentration of tryptophan in the medium
is very high, the optimal concentration of tryptophan is
maintained by a modulating system in which the enzyme
concentration varies with the concentration of tryptophan.
This modulation is effected by
F I G U R E 2 6 -2
Three states of the lac operon showing that lac mRNA is made only if
cAMP-CRP is present and repressor is absent.
five steps, each requiring a particular enzyme. In
E. coli,
these enzymes are translated from a single polycistronic
mRNA. Adjacent to the enzyme coding sequences in the
DNA are a promoter, an operator, and two regions called
the
leader
and the
attenuator
(Figure 26-3). The leader
and attenuator sequences are transcribed. Another gene
(,
trpR
) encoding a repressor is located some distance from
this gene cluster.
Regulation of the trp operon is determined by the con-
centration of tryptophan: when adequate tryptophan is
present in the growth medium, there is no need for tryp-
tophan biosynthesis. Transcription is turned off when a
high concentration of tryptophan is present and is turned
on when tryptophan is absent. The regulatory signal is
the concentration of tryptophan itself and, in contrast
with lactose, tryptophan is active in repression rather than
induction.
The trp operon has two levels of regulation—an on-off
mechanism and a modulation system. The protein product
of the
trpR
gene—the trp aporepressor—cannot bind to
the operator in contrast with the lac repressor. However, if
tryptophan is present, the aporepressor and the tryptophan
molecule join together to form an active repressor complex
1. Premature termination of transcription before the first
structural gene is reached, and
2. Regulation of the frequency of premature termination
by the concentration of tryptophan.
Located between the 5' end of the trp mRNA molecule
and the start codon of the
trpE
gene is a 162-base segment
called the
leader
(a general term for such regions). Within
the leader is a sequence of bases (bases 123 through 150)
having regulatory activity. After initiation of mRNA syn-
thesis, most mRNA molecules are terminated in this region
(except in the complete absence of tryptophan), yielding
a short RNA molecule consisting of only 40 nucleotides
and terminating before the structural genes of the operon.
This region in which termination occurs is a regulatory
region called the
attenuator.
The base sequence around
which termination occurs (Figure 26-4) has the usual fea-
tures of a transcription termination site—namely, a pos-
sible stem-and-loop configuration in the mRNA followed
by a sequence of eight AU pairs.
The leader sequence has an AUG codon that is in-phase
with a UGA stop codon; together these start-stop signals
encode a polypeptide of 14 amino acids. The leader se-
quence has an interesting feature—at positions
1 0
and
1 1
are two adjacent tryptophan codons.
Premature termination of mRNA synthesis is mediated
through translation of the leader peptide. The two tryp-
tophan codons make translation of the leader polypeptide
N u m b e r o f
b a s e p a ir s
60
162
1560
1593
1350
1196
804
300
t
î*
^
I
* l
t
1
1
1
i
-l
p
°
t
v
Attenuator
sequence
(trp a)
trpE
S p a c e r
trpD
trpC
trpB
trpA
Î
S p a c e r
Regulation
E n z y m e p r o d u c tio n
F I G U R E 2 6 -3
E sch erich ia c o li
trp operon. For clarity, the regulatory region is enlarged with respect to the coding region. The proper
size of each region is indicated by the number of base pairs. L is the leader.
