chapter, we discuss this mode of control and its
use by bacteria. The means employed to coor-
dinate control
of dispersed eukaryotic
genes
are
discussed in
Chapter
25, Activating
Tfanscription.
and
galactose (which
are then further
metabolized). This enzyme
also
produces
an important
by-product,
p-1,
6-allo-
lactase,
which
has
a role in regulation.
.
lacY
codes
for
the
p-galactoside
perme-
ase, a l0-kD membrane-bound
protein
constituent of the transport system. This
transports
B-galactosides
into
the cell.
.
lacA codes for
B-galactoside
transacety-
lase, an enzyme that transfers
an acetyl
group
from
acetyl-CoA to
B-galactosides.
Mutations in either lacZ or lacY
can create
the lac
genolype,
in
which cells cannot utilize lac-
tose.
(The genotlpic
description
"lac"
withoul a
qualifier
indicates loss-of-funclion.\
Ti;re lacZ
mutations
abolish enzyme activity, directly
pre-
venting
metabolism
of lactose. The /acYmutants
cannot take up lactose from
the medium.
(No
defect
is
identifiable in lacA cells,
which
is
puzding.
It is
possible
that the acetylation reaction
gives
an advantage when the bacteria
grow
in
the
pres-
ence of certain analogs of
B-galactosides
that can-
not be metabolized,
because the modification
results in
detoxification and excretion.)
The
entire system,
including
structural
genes
and the elements that control
their expres-
sion, forms a
common unit of regulation
called
an operon. The
activity of the operon is
con-
trolled by
regulator
gene(s)
whose
protein prod-
ucts
interact
with the as-acting
control elements.
The
lqc Genes
Are
Contro[Led by a
Repressor
Transcription of
the
lacZYA
gene
cluster is
controlled by a repressor
protein
that binds to an
operator
that overlaps the
promoter
at the start of
the ctuster.
The
repressor
protein
is a
tetramer of identical
subunits coded by the
gene
lacf.
;:.:{ii"!i:i,,r
.1 ..r.,,, The
lacoperon
occupies
-6000
bp of DNA. Atthe
Leftthe laclgene has its
own
pro-
moter
and
terminator. The
end of the locl region is
adjacent to the
promoter,
P. The
operator,
0
occupies the first 26
bp of the transcription
unit. The long IacZ
gene
starts at base 39, and is fot-
lowed
bv the LacY and lac,4
qenes
and a terminator.
@
Structural Gene
Clusters
Are
CoordinateLy
ControLLed
r
Genes coding for
proteins
that
function in
the
same
pathway
may
be [ocated adjacent to one
another and
controlted as a singte unit that
is
transcribed into
a
polycistronic
mRNA.
Bacterial
structural
genes
are often organized
into
clusters that include
genes
coding
for
pro-
teins whose functions
are related. It is common
for
the
genes
coding for the
enzymes of a meta-
bolic
pathway
to be organized into such a clus-
ter. In
addition to the enzymes actually involved
in
the
pathway,
other related
activities
may
be
included
in the
unit of coordinate control; for
example,
the
protein
responsible for
transport-
ing
the small molecule
substrate into the cell.
The
cluster of the three lac
structural
genes,
lacZYA,
is typical.
'i,rlj:ri.
i..lr'i.
gsppurizes
the
organization of the
structural
genes,
their asso-
ciated cli-acting regulatory
elements, and the
trans-acting regulatory
gene.
The key
feature
is
that
the cluster is transcribed
into a single
polycistrontc
wRNA
from
a
promoter
where initiation
of
transcrip-
tion is regulated.
The
protein products
enable
cells to take
up and
metabolize
B-galactosides,
such as lac-
tose. The roles
of the three structural
genes
are:
.
lacZ codes for
the enzyme
B-galactosi-
dase,
whose
active form is a tetramer
of
-500
kD. The
enzyme breaks a
B-galac-
toside into its
component
sugars.
For
example, lactose
is cleaved into
glucose
304
CHAPTER 12 The
0peron