Biochemical and functional characterization of the SMC holocomplex from Mycobacterium smegmatis

Multi-subunit SMC complexes are required to perform essential functions, such as chromosome compaction, segregation and DNA repair, from bacteria to humans. Prokaryotic SMC proteins form complexes with two non-SMC subunits, ScpA and ScpB, to condense the chromosome. The mutants of both scpa and scpb genes in Bacillus subtilis have been shown to display characteristic phenotypes such as growth defects and increased frequency of anucleate cells. Here, we studied the function of the Smc-ScpAB complex from Mycobacterium smegmatis. We observed no significant growth difference between the scpb null mutant and wild-type M. smegmatis under both standard and stress conditions. Furthermore, we characterized the Smc-ScpAB holocomplex from M. smegmatis. The MsSMC consists of the dimerization hinge and ATPase head domains connected by long coiled-coils. The MsSMC interacts with two non-SMC proteins, ScpA and ScpB, and the resulting holocomplex binds to different DNA substrates independent of ATP. The Smc-ScpAB complex showed DNA-stimulated ATPase activity in the presence of ssDNA. A cytological profiling assay revealed that upon overexpression the Smc-ScpAB ternary complex compacts the decondensed nucleoid of rifampicin-treated wild-type and null mukb mutant of Escherichia coli in vivo. Together, our study suggests that M. smegmatis has a functional Smc-ScpAB complex capable of DNA binding and condensation. Based on our observations, we speculate that the presence of alternative SMCs such as MksB or other SMC homologues might have rescued the scpb mutant phenotype in M. smegmatis.