To better understand the contributions that the structural maintenance of 
chromosome proteins (SMCs) make to condensin activity, we have tested a number 
of biochemical, biophysical, and DNA-associated attributes of the Smc2p-Smc4p 
pair from budding yeast. Smc2p and Smc4p form a stable heterodimer, the "Smc2/4 
complex," which upon analysis by sedimentation equilibrium appears to reversibly 
self-associate to form heterotetramers. Individually, neither Smc2p nor Smc4p 
hydrolyzes ATP; however, ATPase activity is recovered by equal molar mixing of 
both purified proteins. Hydrolysis activity is unaffected by the presence of 
DNA. Smc2/4 binds both linearized and circular plasmids, and the binding appears 
to be independent of adenylate nucleotide. High mole ratios of Smc2/4 to plasmid 
promote a geometric change in circular DNA that can be trapped as knots by type 
II topoisomerases but not as supercoils by a type I topoisomerase. Binding 
titration analyses reveal that two Smc2/4-DNA-bound states exist, one disrupted 
by and one resistant to salt challenge. Competition-displacement experiments 
show that Smc2/4-DNA-bound species formed at even high protein to DNA mole 
ratios remain reversible. Surprisingly, only linear and supercoiled DNA, not 
nicked-circular DNA, can completely displace Smc2/4 prebound to a labeled, 
nicked-circular DNA. To explain this geometry-dependent competition, we present 
two models of DNA binding by SMCs in which two DNA duplexes are captured within 
the inter-coil space of an Smc2/4 heterodimer. Based on these models, we propose 
a DNA displacement mechanism to explain how differences in geometry could affect 
the competitive potential of DNA.