by
the simple extension
of a
polymeric
struc-
ture. Another enzyme
is responsible for
gener-
ating
the
peptidoglycan in the
septum
(see
Section
17.5,FtsZIs Necessary
for
Septum
For-
mation). The septum
initially forms as a double
layer of
peptidoglycan,
and the
protein
EnvA
is
required to split the covalent
links between the
layers
so that
the daughter cells may separate.
The behavior of
the
periseptal
annulus sug-
gests
that
the mechanism
for
measuring
posi-
tion is associated with
the cell envelope. It is
plausible
to suppose that the envelope could
also be used
to ensure segregation of the chro-
mosomes.
A
direct
link between DNA and the
membrane could account
for
segregation.
If
daughter chromosomes
are attached to the
membrane, they could be
physically
separated
when the septum
forms.
Fl+#F{
1}"s+ shows that
the formation of a septum could segregate the
chromosomes
into the different daughter cells
if the origins are connected
to
sites
that lie
on
either
side of the
periseptal
annulus.
i:i.:i:i:::l
':
..-
Attachment of bacteriaI DNA to the mem-
brane could
provide
a mechanism for segregation.
the correct
position
may be the crucial event
that ensures the division of the cell into daugh-
ters
of equal size.
(The
mechanism
of
move-
ment is
unknown.) Septation begins when the
cell reaches a fixed length
(2L),
and the distance
between the new annuli
is
always L. We do
not
know how the cell measures length, but the
relevant
parameter
appears to
be
linear distance
as such
(not
area
or
volume).
The
septum consists of the same compo-
nents
as the cell envelope: There is a rigid layer
of
peptidoglycan
in
the
periplasm,
between
the inner
and outer membranes. The
peptido-
glycan
is made by
polymerization
of tri- or
pentapeptide-disaccharide
units in a reaction
involving connections
between both types of
subunit
(transpeptidation
and transglycosyla-
tion). The rodlike
shape of the bacterium
is
maintained
by a
pair
of activities, PBP2 and
RodA. They
are interacting
proteins
and are
coded
by the same operon. RodA is a member
of the
SEDS
family
(SEDS
stands for
shape,
elon-
gation,
division, and
sporulation) that is
pres-
ent in
all bacteria that have a
peptidoglycan
cell
wall. Each SEDS
protein
functions
together with
a
specific transpeptidase,
which catalyzes the
formation
of the crosslinks in
the
peptidoglycan.
PBP2
(penicillin-binding
protein
2) is the
transpeptidase
that interacts with RodA. Muta-
tions in the
gene
for
either
protein
cause the
bacterium
to Iose its extended
shape and become
round. This
demonstrates the important
prin-
ciple that shape and rigidity
can be determined
CHAPTER 17 Bacterial
Reptication
Is Connected to the
Ce[[ Cycte
Origins of
replicating
chromosomes
attached
to membrane
Daughter chromosomes
attached to envelope
Septum
grows
between chromosomes
Septum
divides cell
Chromosomes
distributed to
daughter cells
Mutations in Division
or Segregation
AfFect
Ce[[ Shape
o
jfs
mutants form long fitaments because the
septum
fails to form to divide the daughter
bacteria.
o
Minicelts form in mutants that
produce
too many
septa;
they are sma[[ and Lack DNA.
o
Anucteate cetts of
normal
size are
generated
by
partition
mutants
in
which the dupticate
chromosomes
faiI
to seoarate.
A difficulty in isolating mutants that affect
cell
division is that mutations
in
the critical functions
may
be
lethal
and/or
pleiotropic.
For example,
if formation of the annulus occurs at a site that
is essential for overall
growth
of the envelope,
it would be difficult to distinguish mutations
that specifically interfere
with
annulus
forma-
tion from those that
inhibit
envelope
growth
generally.
Most mutations in
the division appa-
ratus have been identified as
conditional
mutants
(whose
division is affected under non-
permissive
conditions; typically they
are tem-
perature
sensitive). Mutations
that affect cell
division or chromosome segregation
cause
striking
phenotypic
changes.
Ftr{itlfito"
l;.*
and
It"$ul{F.
i,r"ii
illustrate the opposite consequences
of
failure
in the division
Drocess
and failure in
segregation:
472