MksB, an alternate condensin from Mycobacterium smegmatis is involved in DNA binding and condensation.

The structural maintenance of chromosomes (SMC) proteins play a vital role in genome stability
and chromosome organization in all domains of life. Previous reports show that smc deletion causes
decondensation of chromosome and an increased frequency of anucleated cells in bacteria. However, smc
deletion in both Mycobacterium smegmatis and Mycobacterium tuberculosis did not affect chromosome
condensation or the frequency of anucleated cells. In an attempt to understand this difference in M.
smegmatis, we investigated the function of MksB (MsMksB), an alternate SMC-like protein. Like other
bacterial SMCs, MsMksB is also an elongated homodimer, in which a central hinge domain connects two
globular ATPase head domains via two coiled-coil arms. We show that full-length MsMksB binds to
different topological forms of DNA without any preferences. However, the hinge and headless domains
prefer binding to single-stranded DNA (ssDNA) and linear double-stranded DNA (dsDNA), respectively.
The binding of MsMksB to DNA was independent of ATP as its ATP hydrolysis deficient mutant was
also proficient in DNA binding. Further, the cytological profiling studies revealed that only the full-length
MsMksB and none of its structural domains could condense the bacterial chromosome. This observation
indicates the plausibility of the concerted action of different structural domains of SMC to bind and
condense the chromosome. Moreover, MsMksB exhibited DNA stimulated ATPase activity, in addition
to its intrinsic ATPase activity. Taken together, we have elucidated the function of an alternate bacterial
condensin protein MksB and its structural domains in DNA binding and condensation.