Thanks to genetics, to the study of protein-protein interactions and t
o direct viewing of subcellular structures by the use of immunofluores
cence and green fluorescent protein (GFP) fusions, the organization of
the constriction apparatus of walled bacteria is gradually coming to
light, The tubulin-like protein FtsZ assembles as a ring around the si
te of constriction and operates as an organizer and activator of septu
m-shaping proteins, Much less is known about the factors specifying th
e location of FtsZ rings, Circumstantial evidence favours the presence
at future ring positions of fixed elements, the potential division si
tes (PDS), before FtsZ assembles. FtsZ polymerization is initiated fro
m a point on a PDS, the nucleation site, still to be identified, and p
roceeds bidirectionally around the cell, We hypothesize that new PDS a
re specified in a manner that depends on the functioning of an active
chromosome partition apparatus, This view is supported by the fact tha
t formation of mid-cell PDS requires initiation of DNA replication, an
d by recent studies supporting the existence of a specialized partitio
n apparatus in a variety of microorganisms, Although PDS may be specif
ied directly by the partition apparatus, indirect localization linked
to compartmentalized gene expression during chromosome segregation is
also possible, Once created, PDS are used in a regulated manner, and s
everal mechanisms normally operate to direct constriction to selected
PDS at the correct time, One, dedicated to the permanent suppression o
f polar PDS, rests on the minicell suppression system and involves a p
rotein that is able to discriminate between polar and non-polar sites,
Another is involved in asymmetric site selection at the early stages
of sporulation in Bacillus subtilis, Finally, a mechanism observed onl
y in certain multinucleated cells appears to favour division at nonpol
ar PDS related to the most ancient replication/DNA segregation events.