Bacterial Growth and Division
541
during the division cycle with constant
rates of synthesis. Proteins are synthesized
during the division cycle with a pattern
that is presumably consistent with their
Fnal location or category. Proteins are not
a monolithic group nor are they divided
into a large number of groups. The three-
category system proposed here allows us to
conceptualize the possible patterns of pro-
tein synthesis observed during the division
cycle.
3.3.2
The Invagination Process
The division of the bacterial cell into two
daughter cells occurs by the formation of
two new poles in the middle of the dividing
cell. As there are very few DNA-less cells
produced during balanced growth, the
division process must be coordinated with
the replication process so that division
does not occur until two genomes are
present. The invagination process that
forms two poles in the middle of the cell
involves a complex process whereby there
is an inward growth of the peptidoglycan
and the associated membrane of the cell
surface.
One of the earliest events related to the
invagination process is the localization of
the ±tsZ protein to the future site of pole
formation. The ±tsZ protein is localized
to the division site in a pattern called the
FtsZ ring
. The ±tsZ protein accumulates at
the mid-cell site where division will occur
before any visible invagination. During
the invagination process, the ±tsZ protein
is located at the leading edge of the
invagination process. Thus, the ±tsZ ring
continues to deFne a ring of decreasing
size as invagination proceeds.
Other proteins necessary for invagina-
tion and division are apparently localized
at the center of the cell because of their
association with ±tsZ.
3.3.3
FtsZ Protein Is Involved in Septum
Formation and Cell Division
Recent results indicate that the tubulin-
like ±tsZ protein plays a central role in
cytokinesis or cell division as a major
component of a contractile cytoskeleton.
An indication that the ±tsZ protein is the
key initiator of septation is that mutants
of this protein that form Flaments show
no indication of any constriction. Other
septation mutants presumed to be later
in the pathway of septum formation do
show some indication of septation. The
±tsZ protein resembles the eukaryotic
tubulin molecule and this similarity has
led to speculations that the ±tsZ protein
has cytoskeletal properties that allow it
to form the new pole in the middle of
the cell.
3.3.4
Localization of the Division Site
The actual mechanism by which the divi-
sion site is localized at mid-cell position is
unknown. One model of the division-site
localization proposes that the site exists
prior to any observation of ±tsZ localiza-
tion. An alternative model proposes that
the division site is determined in some
way by the reaction of the cell surface to
the presence of DNA; the absence of DNA
in the center of the cell would thus lead to
invagination-site localization.
An understanding of site location is
strongly influenced by the existence of
minicells. If invagination occurs, upon oc-
casion, near a pole rather than in the center
of a cell, a very small DNA-less ‘‘mini-
cell’’ is produced that is essentially two
poles without an intervening sidewall. The
genetics of minicell formation is compli-
cated, but the main idea is that the products
of the min genes have a negative function
and normally inhibit abnormal invagina-
tion sites. Thus, inactivation of any of a
number of min genes leads to minicell
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