A new family of bacterial condensins

Abstract

Condensins play a central role in global chromatin organization. In bacteria, two families of condensins have been identified, the MukBEF and SMC-ScpAB complexes. Only one of the two complexes is usually found in a given species, giving rise to a paradigm that a single condensin organizes bacterial chromosomes. Using sequence analysis, we identified a third family of condensins, MksBEF (MukBEF-like SMC proteins), which is broadly present in diverse bacteria. The proteins appear distantly related to MukBEF, have a similar operon organization and similar predicted secondary structures albeit with notably shorter coiled coils. All three subunits of MksBEF exhibit significant sequence variation and can be divided into a series of overlapping subfamilies. MksBEF often coexists with the SMC-ScpAB, MukBEF and, sometimes, other MksBEFs. In Pseudomonas aeruginosa, both SMC and MksB contribute to faithful chromosome partitioning, with their inactivation leading to increased frequencies of anucleate cells. Moreover, MksBEF can complement anucleate cell formation in SMC-deficient cells. Purified PaMksB showed activities typical for condensins including ATP-modulated DNA binding and condensation. Notably, DNA binding by MksB is negatively regulated by ATP, which sets it apart from other known SMC proteins. Thus, several specialized condensins might be involved in organization of bacterial chromosomes.

Pseudomonas aeruginosa Condensins Support Opposite Differentiation States

“…Mutational inactivation of SMC results in low-frequency anucleate cell formation and chromosome packing defects (28,29). We report here that both condensins are expendable under laboratory conditions in P. aeruginosa but are needed for the differentiation of the bacterium.…”

“…MksB and MksEF were purified as previously described (28). SMC-His 8 was cloned into the pBADN plasmid, overproduced.…”

“…PAO1-Lac (ATCC 47085) was used as the wild-type (WT) strain unless otherwise noted. Mutations were introduced using allele replacement as described earlier for OP103 (28), followed by the excision, when indicated, of the gentamicin resistance marker (Gm) with the help of the pFLP2 plasmid (30). Deletions of sspB were constructed using the pEXG2-based plasmids as described elsewhere (31).…”

“…Microscopy was performed as previously described (28). Cell sizes were quantified using the program Nucleus (36).…”

“…The ATPase mutations of MksB (strains OP117) included D864A (DA), E865Q (EQ), and S829R (SR). These mutations were constructed using overlap extension PCR followed by the insertion of the fragment into plasmid pEX_⌬mksB (28) between the KpnI and BlpI sites. This was followed by the integration of the resulting mutants onto the endogenous mksB locus of the chromosome of PAO1.…”