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chapter 26
Regulation of Gene Expression
Some bacterial operons are regulated solely by attenua-
tion without repressor-operator interactions.
Temporal mRNA Regulation in Phage Systems
The lac and trp operons are regulated by molecules that
turn gene expression on and off in response to the con-
centration of nutrients. However, some organisms require
regulation of genes in time. For example, in the produc-
tion of phage (bacteriophage or bacterial viruses) in an
infected cell, various biosynthetic activities must occur on
schedule. When a bacterium is infected by a phage, the en-
zymes responsible for breaking open the cell and releasing
progeny phage must act late in the life cycle after phage
DNA replication and packaging has occurred; otherwise,
the cell could burst before any phage formed. Almost all
phage systems accomplish this temporal regulation by se-
lective and sequential transcription of particular sets of
genes. This transcription gives rise to various classes of
mRNA which are usually termed
early
and
late mRNA.
Three
E. coli
phage systems, which accomplish temporal
regulation in different ways, are described briefly below.
Phage T7
contains several promoters, but only one
is recognized by
E. coli
RNA polymerase. Transcription
of T7 DNA begins at this promoter. Two important pro-
teins are encoded there by transcript (early mRNA). One
phosphorylates and inactivates the
E. coli
RNA poly-
merase, thus preventing
E. coli
from making any bac-
terial mRNA. The second protein is a new RNA poly-
merase that does not recognize any
E. coli
promoters but is
active at the remaining phage promoters. In the absence
of
E. coli
RNA polymerase, the early mRNA is no longer
made, but the new T7 RNA polymerase makes the second
transcript, which encodes the DNA replication enzymes
and other proteins needed early in the life cycle. Tran-
scription at the third promoter is delayed because injection
of the phage DNA into the bacterium occurs very slowly.
Roughly halfway through the life cycle of the phage this
promoter enters the cell, transcription occurs by T7 RNA
polymerase, and the structural proteins and lysis enzymes
are synthesized. Lysis does not occur until phage particles
have been assembled.
Phage T4
has numerous promoters only a few of which
can be recognized by
E. coli
RNA polymerase. However,
unlike T7, the late promoters are made available by succes-
sive modification of the
E. coli
enzyme. These modifica-
tions are of two types: addition of phage-encoded protein
subunits and chemical modification of preexisting sub-
units. Temporal regulation occurs because the gene re-
sponsible for the first modification is encoded in the first
set of mRNAs, that for the second modification in the sec-
ond set, and so on. To ensure that the late mRNA, which
encodes the structural proteins and the lysis enzyme, is
not synthesized until adequate DNA has been made by the
replication system, the template for this late mRNA cannot
be parental T4 DNA but must be a replica.
E. coli phage A
also uses
E. coli
RNA polymerase
throughout its life cycle. With this phage, regulation of
transcription is accomplished in two ways: modification
of the positive regulatory elements, such as the cAMP-
CRP complex, needed for recognition of certain promot-
ers; specific repressors are also used to turn off expression
of certain genes. Figure 26-6 shows a portion of the ge-
netic map of
k,
which includes four regulatory genes (
cro
,
N, Q,
and
ell),
three promoters (pL, pR, and pR2), and
five termination sites (tLl, tRl, tR2, tR3, and tR4). Seven
mRNA molecules are also shown; the L and R series are
transcribed leftward and rightward respectively from com-
plementary DNA strands. The genes
O
and
P
encode pro-
teins required for
k
DNA replication, and the late genes
encode the structural proteins of the phage and the lysis
enzyme. The basic transcription sequence is: Two early
mRNAs, LI and RI, are made that encode the regulatory
proteins N and Cro, respectively. N is an antitermination
a f t
int
red
i
c/l
ro
i
cro
D N A
i
ell
'o p '
I
o
L a te g e n e s
tL1
nutL pL oL
LI
oR
pR
nutR tR1
R1
tR2
!R3
pR2
tR4
(■
qut)
L2
R 2
~~y
—
R 3
R 4
v________
v
j
F I G U R E 2 6 - 6
Genetic map of the regulatory genes of phage
X.
Genes are listed above the line; sites are below the line. The mRNA
molecules are heavy lines. The dashed black arrows with thin lines indicate the sites of action of the N, Cro, and Q
proteins.
