In C. crescentus(Figure 1d), the origin is located at the flagellated pole in the motile swarmer cells and the terminus
is at the opposite end of the cell. DNA replication initiates
only after the swarmer cell differentiates into a non-motile
stalked cell, by shedding the flagellum and synthesizing a
stalk at the same pole. One of the newly replicated origins
then immediately moves to the opposite pole and the terminus moves to mid-cell [13•
].
Time-lapse microscopy of chromosome movement in living E. colior B. subtiliscells reveals that the origins separate
rapidly, indicating the presence of an active mechanism
involved in the movement [8,11
•
].
Chromosomal partitioning proteins
Chromosomal homologs of the partitioning genes parAand
parBare found in many bacteria. In B. subtilis, they are
named soj and spo0J, respectively, and their role in chromosome segregation is suggested by the observation that
deletion of spo0J results in an increase in the number of
anucleate cells [3]. Localization of Spo0J using immunofluorescence microscopy or GFP fusions revealed that it
forms discrete nucleoid-associated foci that are usually
located towards the cell poles or mid-cell. This localization
pattern is similar to the localization of the origins of replication [12,14–17], and co-localization experiments
confirmed that Spo0J does indeed localize to the same
sites as the origin [12,16]. Spo0J is a site-specific DNAbinding protein that binds to at least eight sites in the
origin–proximal 20% of the chromosome [18••
]. Plasmids
containing an Spo0J-binding site segregate more efficiently to the daughter cells when sojandspo0Jare expressed.
This active segregation of plasmids conferred by the Spo0J
binding site indicates that the soj and spo0J genes have
direct functions in chromosome segregation and that the
Spo0J-binding sites are centromere-like partitioning sites.
The Caulobacterchromosomal parAand parBgenes are
essential for viability, and overexpression of these proteins
causes chromosome segregation defects [4]. ParA and ParB
localize to the poles in pre-divisional cells, and ParB binds
to a region of the chromosome close to the origin of replication. Therefore, these proteins also appear to be
involved in chromosome segregation.
Systems other than the chromosomal partitioning genes are
involved in chromosome segregation, however, since some
bacteria (e.g.E. coli) do not contain chromosomal parAand
parBgenes, and the same pattern of origin localization and
movement is observed in most cells in a B. subtilisstrain in
which both soj andspo0J are deleted [11
•
].
The DNA replication apparatus in B. subtilisis anchored
at discrete intracellular positions, predominantly at midcell [19
••
]. Thus, replicating DNA is spooled through a
stationary replication machine at mid-cell, and this may
force the newly replicated chromosomes away from each
other [20]. It is unknown how the polar origins are replicated; a polar DNA polymerase subunit could be
responsible for replication of the origin–proximal DNA or
the DNA polymerase subunits could be recruited to midcell after initiation of DNA replication. Extrusion of
newly replicated DNA from the replication machinary
followed by nucleoid rebuilding, could be involved in
establishing the bipolar organization of the origins, and
the primary function of the chromosomal partitioning