Mechanistic study of classical translocation-dead SpoIIIE36 reveals the functional importance of the hinge within the SpoIIIE motor

A new role for monomeric ParA/Soj in chromosome dynamics in Bacillus subtilis

ParABS (Soj-Spo0J) systems were initially implicated in plasmid and chromosome segregation in bacteria. However, it is now increasingly understood that they play multiple roles in cell cycle events in Bacillus subtilis, and possibly other bacteria. In a recent study, monomeric forms of ParA/Soj have been implicated in regulating aspects of chromosome dynamics during B. subtilis sporulation. In this commentary, I will discuss the known roles of ParABS systems, explore why sporulation is a valuable model for studying these proteins, and the new insights into the role of monomeric ParA/Soj. Finally, I will touch upon some of the future work that remains.

Chromosome Segregation and Peptidoglycan Remodeling Are Coordinated at a Highly Stabilized Septal Pore to Maintain Bacterial Spore Development

Asymmetric division, a hallmark of endospore development, generates two cells, a larger mother cell and a smaller forespore. Approximately 75% of the forespore chromosome must be translocated across the division septum into the forespore by the DNA translocase SpoIIIE. Asymmetric division also triggers cell-specific transcription, which initiates septal peptidoglycan remodeling involving synthetic and hydrolytic enzymes. How these processes are coordinated has remained a mystery. Using Bacillus subtilis, we identified factors that revealed the link between chromosome translocation and peptidoglycan remodeling. In cells lacking these factors, the asymmetric septum retracts, resulting in forespore cytoplasmic leakage and loss of DNA translocation. Importantly, these phenotypes depend on septal peptidoglycan hydrolysis. Our data support a model in which SpoIIIE is anchored at the edge of a septal pore, stabilized by newly synthesized peptidoglycan and protein-protein interactions across the septum. Together, these factors ensure coordination between chromosome translocation and septal peptidoglycan remodeling to maintain spore development.

Cohesion of Sister Chromosome Termini during the Early Stages of Sporulation in Bacillus subtilis

During sporulation of Bacillus subtilis, the cell cycle is reorganized to generate separated prespore and mother cell compartments, each containing a single fully replicated chromosome. The process begins with reorganization of the nucleoid to form an elongated structure, the axial filament, in which the two chromosome origins are attached to opposite cell poles, with the remainder of the DNA stretched between these sites. When the cell then divides asymmetrically, the division septum closes around the chromosome destined for the smaller prespore, trapping the origin-proximal third of the chromosome in the prespore. A translocation pore is assembled through which a DNA transporter, SpoIIIE/FtsK, transfers the bulk of the chromosome to complete the segregation process. Although the mechanisms involved in attaching origin regions to the cell poles are quite well understood, little is known about other aspects of axial filament morphology. We have studied the behavior of the terminus region of the chromosome during sporulation using time-lapse imaging of wild-type and mutant cells. The results suggest that the elongated structure involves cohesion of the terminus regions of the sister chromosomes and that this cohesion is resolved when the termini reach the asymmetric septum or translocation pore. Possible mechanisms and roles of cohesion and resolution are discussed.IMPORTANCE Endospore formation in Firmicutes bacteria provides one of the most highly resistant life forms on earth. During the early stages of endospore formation, the cell cycle is reorganized so that exactly two fully replicated chromosomes are generated, before the cell divides asymmetrically to generate the prespore and mother cell compartments that are critical for the developmental process. Decades ago, it was discovered that just prior to asymmetrical division the two chromosomes enter an unusual elongated configuration called the axial filament. This paper provides new insights into the nature of the axial filament structure and suggests that cohesion of the normally separated sister chromosome termini plays an important role in axial filament formation.

Sequence-dependent catalytic regulation of the SpoIIIE motor activity ensures directionality of DNA translocation

Transport of cellular cargo by molecular motors requires directionality to ensure proper biological functioning. During sporulation in Bacillus subtilis, directionality of chromosome transport is mediated by the interaction between the membrane-bound DNA translocase SpoIIIE and specific octameric sequences (SRS). Whether SRS regulate directionality by recruiting and orienting SpoIIIE or by simply catalyzing its translocation activity is still unclear. By using atomic force microscopy and single-round fast kinetics translocation assays we determined the localization and dynamics of diffusing and translocating SpoIIIE complexes on DNA with or without SRS. Our findings combined with mathematical modelling revealed that SpoIIIE directionality is not regulated by protein recruitment to SRS but rather by a fine-tuned balance among the rates governing SpoIIIE-DNA interactions and the probability of starting translocation modulated by SRS. Additionally, we found that SpoIIIE can start translocation from non-specific DNA, providing an alternative active search mechanism for SRS located beyond the exploratory length defined by 1D diffusion. These findings are relevant in vivo in the context of chromosome transport through an open channel, where SpoIIIE can rapidly explore DNA while directionality is modulated by the probability of translocation initiation upon interaction with SRS versus non-specific DNA.

Missense Mutations Allow a Sequence-Blind Mutant of SpoIIIE to Successfully Translocate Chromosomes during Sporulation

SpoIIIE directionally pumps DNA across membranes during Bacillus subtilis sporulation and vegetative growth. The sequence-reading domain (γ domain) is required for directional DNA transport, and its deletion severely impairs sporulation. We selected suppressors of the spoIIIEΔγ sporulation defect. Unexpectedly, many suppressors were intragenic missense mutants, and some restore sporulation to near-wild-type levels. The mutant proteins are likely not more abundant, faster at translocating DNA, or sequence-sensitive, and rescue does not involve the SpoIIIE homolog SftA. Some mutants behave differently when co-expressed with spoIIIEΔγ, consistent with the idea that some, but not all, variants may form mixed oligomers. In full-length spoIIIE, these mutations do not affect sporulation, and yet the corresponding residues are rarely found in other SpoIIIE/FtsK family members. The suppressors do not rescue chromosome translocation defects during vegetative growth, indicating that the role of the γ domain cannot be fully replaced by these mutations. We present two models consistent with our findings: that the suppressors commit to transport in one arbitrarily-determined direction or delay spore development. It is surprising that missense mutations somehow rescue loss of an entire domain with a complex function, and this raises new questions about the mechanism by which SpoIIIE pumps DNA and the roles SpoIIIE plays in vivo.

Do the same traffic rules apply? Directional chromosome segregation by SpoIIIE and FtsK

Over a decade of studies have tackled the question of how FtsK/SpoIIIE translocases establish and maintain directional DNA translocation during chromosome segregation in bacteria. FtsK/SpoIIIE translocases move DNA in a highly processive, directional manner, where directionality is facilitated by sequences on the substrate DNA molecules that are being transported. In recent years, structural, biochemical, single-molecule and high-resolution microscopic studies have provided new insight into the mechanistic details of directional DNA segregation. Out of this body of work, a series of models have emerged and, ultimately, yielded two seemingly opposing models: the loading model and the target search model. We review these recent mechanistic insights into directional DNA movement and discuss the data that may serve to unite these suggested models, as well as propose future directions that may ultimately solve the debate.