Bacillus subtilis Smc condenses chromosomes in a heterologous cell system, which is down-regulated by ScpAB

Structural maintenance of chromosomes (SMC) proteins are key players in chromosome dynamics in all types of organisms. The so-called condensin subfamily is essential for chromosome condensation in eukaryotic cells, as is the bacterial SMC complex (called MukBEF in Escherichia coli). We expressed the Bacillus subtilis Smc protein and its two complex partners ScpA and ScpB in E. coli cells, and monitored effects on chromosome compaction by DNA staining of live cells using epifluorescence microscopy.

We show that expression of BsSmc leads to strong chromosome compaction, while expression of ScpAB does not show any effect. Chromosome compaction by Smc was also found for mutant versions lacking ATP binding or ability for head engagement, and was counteracted by concomitant expression of ScpAB. Our findings show that the SMC complex can act as autonomous condensation system in a heterologous bacterial host system, for which neither ATP binding nor ATP hydrolysis are required. Our investigation suggests that the negative effect on compaction activity of Smc exerted by ScpAB in vivo does not involve an effect on ATPase activity, but more likely a stabilization of the engagement of head domains, which in turn may affect ATPase activity.