Figure 3: Biochemically distinct condensin
complexes with interchangeable subunits
control chromosome structure
throughout C. elegans development. a,
Each condensin complex contains a pair of SMC
proteins (Structural Maintenance of
Chromosomes) and three non-SMC subunits.
SMC proteins have nucleotide binding domains
at their globular N- and C-termini, which are
linked by two long coiled coil domains separated
by a hinge region. The dosage compensation
condensin complex (condensin IDC) specifically
changes the molecular topology of interphase X
chromosomes in XX embryos to repress gene
expression by half. b, An XX embryo co-stained
with a DNA dye and the DCC-specific subunit
DPY-27 shows X-specific localization of the DCC.
Condensin I and II perform independent roles in
the structure and segregation of mitotic and
meiotic chromosomes, with condensin II being
the primary regulator of chromosome
compaction and resolution (see also h). Both
condensin I and II affect the number and
distribution of double strand breaks in meiosis,
and thereby the number and position of
crossovers between homologous chromosomes,
by controlling the higher-order chromosome
structure. c, Confocal images show that
mutations disrupting condensin I and II subunits
cause an additive extension of chromosome
axes. Meiotic X-chromosome axes from paired
and synapsed homologs in pachytene nuclei of
wild-type and condensin-mutant gonads were
stained with antibodies to the axial protein
HTP-3, traced in 3 dimensions, and
computationally straightened. Axial length is
shown.
Condensin II co-localizes with centromere
proteins and mediates faithful chromosome
segregation. d, Two-celled, wild-type embryo
stained with DNA dye (blue) and antibodies to
tubulin (green) and the centromere-specific
histone HCP-3 (red). The cell on the left is in
metaphase of mitosis. e, One-celled, wild-type
embryo in metaphase stained with DNA dye
(blue) and antibodies to tubulin (green) and the
condensin II subunit SMC-4 (red). f, Enlargement
of mitotic chromosomes in metaphase
co-stained with DNA dye (blue) and antibodies to
SMC-4 (green) and HCP-3 (red). The merged
image (bottom) shows the colocalization of
centromere proteins and condensin II on the
holocentric nematode chromosomes.
DNA-stained wild-type (g) and smc-4(-) (h)
embryos after segregation of homologs (polar
body, green arrow) and sister chromatids (polar
body, white arrow) and migration and fusion of
the sperm (s) and oocyte (o) pronuclei. In
smc-4(-) embryos the oocyte pronucleus remains
connected to the sister chromatids in the polar
body, demonstrating improper chromosome
segregation in meiosis. The dosage
compensation subunit MIX-1 is shared with
condensin II and behaves similarly to SMC-4 in
mitosis.