Proper chromatin condensation and sister chromatid resolution are essential for 
the maintenance of chromosomal integrity during cell division, and is in part 
mediated by a conserved multisubunit apparatus termed the condensin complex. The 
core subunits of the complex are members of the SMC2 (Structural Maintenance of 
Chromosomes) and SMC4 gene families. We have cloned an Arabidopsis gene, 
AtCAP-E1, which is a functional ortholog of the yeast SMC2 gene. A second, 
highly homologous SMC2 gene, AtCAPE-2, was identified by the Arabidopsis genome 
project. SMC2 gene expression in Arabidopsis was correlated with the mitotic 
activity of tissues, with high level expression observed in meristematic cells. 
The two genes are differentially expressed with AtCAP-E1 accounting for more 
than 85% of the total SMC2 transcript pool. The titan3 mutant is the result of a 
T-DNA insertion into AtCAP-E1, but other than subtle endosperm defects, titan3 
is viable and fecund. We identified a T-DNA insertion mutant of AtCAP-E2, which 
showed no obvious mutant phenotype, indicating that the two genes are 
functionally redundant. Genetic crosses were employed to examine the 
consequences of reduced SMC2 levels. Both male and female gametogenesis were 
compromised in double mutant spores. Embryo lethality was observed for both 
double homozygous and AtCAP-E1(-/-), AtCAP-E2(+/-) plants; arrest occurred at or 
before the globular stage and was associated with altered planes of cell 
division in both the suspensor and the embryo. Down regulation of both genes by 
antisense technology, as well as in AtCAP-E1(+/-), AtCAP-E2(-/-) plants results 
in meristem disorganization and fasciation. Our data are consistent with the 
interpretation that threshold levels of SMC2 proteins are required for normal 
development and that AtCAP-E2 may have a higher affinity for its target than 
AtCAP-E1.