Cell division gene cluster in Spiroplasma kunkelii: functional characterization of ftsZ and the first report of ftsA in mollicutes

Dynamics of interdomain rotation facilitates FtsZ filament assembly

FtsZ, the tubulin homolog essential for bacterial cell division, assembles as the Z-ring at the division site, and directs peptidoglycan synthesis by treadmilling. It is unclear how FtsZ achieves kinetic polarity that drives treadmilling. To obtain insights into fundamental features of FtsZ assembly dynamics independent of peptidoglycan synthesis, we carried out structural and biochemical characterization of FtsZ from the cell wall-less bacteria, Spiroplasma melliferum (SmFtsZ). Interestingly the structures of SmFtsZ, bound to GDP and GMPPNP respectively, were captured as domain swapped dimers. SmFtsZ was found to be a slower GTPase with a higher critical concentration (CC) compared to Escherichia coli FtsZ (EcFtsZ). In FtsZs, a conformational switch from R-state (close) to T-state (open) favors polymerization. We identified that Phe224, located at the interdomain cleft of SmFtsZ, is crucial for R- to T-state transition. SmFtsZF224M exhibited higher GTPase activity and lower CC, whereas the corresponding EcFtsZM225F resulted in cell division defects in E. coli. Our results demonstrate that relative rotation of the domains is a rate-limiting step of polymerization. Our structural analysis suggests that the rotation is plausibly triggered upon addition of a GTP-bound monomer to the filament through interaction of the preformed N-terminal domain (NTD). Hence, addition of monomers to the NTD-exposed end of filament is slower in comparison to the C-terminal domain (CTD) end, thus explaining kinetic polarity. In summary, the study highlights the importance of interdomain interactions and conformational changes in regulating FtsZ assembly dynamics.

Exploring Spiroplasma Biology: Opportunities and Challenges

Spiroplasmas are cell-wall-deficient helical bacteria belonging to the class Mollicutes. Their ability to maintain a helical shape in the absence of cell wall and their motility in the absence of external appendages have attracted attention from the scientific community for a long time. In this review we compare and contrast motility, shape determination and cytokinesis mechanisms of Spiroplasma with those of other Mollicutes and cell-walled bacteria. The current models for rod-shape determination and cytokinesis in cell-walled bacteria propose a prominent role for the cell wall synthesis machinery. These models also involve the cooperation of the actin-like protein MreB and FtsZ, the bacterial homolog of tubulin. However the exact role of the cytoskeletal proteins is still under much debate. Spiroplasma possess MreBs, exhibit a rod-shape dependent helical morphology, and divide by an FtsZ-dependent mechanism. Hence, spiroplasmas represent model organisms for deciphering the roles of MreBs and FtsZ in fundamental mechanisms of non-spherical shape determination and cytokinesis in bacteria, in the absence of a cell wall. Identification of components implicated in these processes and deciphering their functions would require genetic experiments. Challenges in genetic manipulations in spiroplasmas are a major bottleneck in understanding their biology. We discuss advancements in genome sequencing, gene editing technologies, super-resolution microscopy and electron cryomicroscopy and tomography, which can be employed for addressing long-standing questions related to Spiroplasma biology.

A structural framework for a near-minimal form of life: mass and compositional analysis of the helical mollicute Spiroplasma melliferum BC3

Spiroplasma melliferum is a wall-less bacterium with dynamic helical geometry. This organism is geometrically well defined and internally well ordered, and has an exceedingly small genome. Individual cells are chemotactic, polar, and swim actively. Their dynamic helicity can be traced at the molecular level to a highly ordered linear motor (composed essentially of the proteins fib and MreB) that is positioned on a defined helical line along the internal face of the cell's membrane. Using an array of complementary, informationally overlapping approaches, we have taken advantage of this uniquely simple, near-minimal life-form and its helical geometry to analyze the copy numbers of Spiroplasma's essential parts, as well as to elucidate how these components are spatially organized to subserve the whole living cell. Scanning transmission electron microscopy (STEM) was used to measure the mass-per-length and mass-per-area of whole cells, membrane fractions, intact cytoskeletons and cytoskeletal components. These local data were fit into whole-cell geometric parameters determined by a variety of light microscopy modalities. Hydrodynamic data obtained by analytical ultracentrifugation allowed computation of the hydration state of whole living cells, for which the relative amounts of protein, lipid, carbohydrate, DNA, and RNA were also estimated analytically. Finally, ribosome and RNA content, genome size and gene expression were also estimated (using stereology, spectroscopy and 2D-gel analysis, respectively). Taken together, the results provide a general framework for a minimal inventory and arrangement of the major cellular components needed to support life.

Structure of the cytoskeleton of Spiroplasma melliferum BC3 and its interactions with the cell membrane

Spiroplasma melliferum is a wall-less bacterium with dynamic helical symmetry. Taking advantage of the simplicity of this primitive lifeform, we have used structural (electron tomography and freeze fracture of whole cells; cryoelectron tomography and diffraction analysis of isolated cytoskeletons) and proteomic approaches to elucidate the basic organizing principles of its minimal yet functional cytoskeleton. From among approximately 30 Spiroplasma proteins present in a highly purified cytoskeletal fraction, we identify three major putative structural proteins: Fib, MreB, and elongation factor Tu. Fib assembles into a single flattened ribbon that follows the shortest helical line just under the plasma membrane and acts as a linear motor, whereas MreB is present as a matching array of membrane-associated fibrils parallel and associated with the motor. We also identify a prominent previously unknown filamentous network that occupies much of the cytoplasm and appears to cross-link the ribosomes. The abundant potentially filament-forming protein elongation factor Tu may be a component of this network, but the tomography data are most consistent with DNA as the core component. The results provide new information on the minimal organization necessary to support the scaffolding and motile functions of a minimal cytoskeleton.

The cytoskeleton of spiroplasma: a complex linear motor

Spiroplasma are wall-less, helical bacteria from the class Mollicutes. The Mollicutes (Mycoplasma, Acholeplasma, Spiroplasma) evolved by regressive evolution to generate one of the simplest and minimal free-living and self-replicating forms of life. The spiroplasmas are the more advanced members in the class and are the closest to their clostridial ancestors. Spiroplasmas were discovered and identified as such only in 1972 and the finding of a unique and well-defined internal cytoskeleton, believed to be uncommon in bacteria, followed in 1973. Structural analysis suggests that the core of the spiroplasmal cytoskeleton is a flat, monolayered ribbon comprised of the 59-kDa fib gene product. The ribbon follows the shortest helical line of the polar cell from end to end. The structural building blocks of the cytoskeletal ribbon are fibrils assembling into a structure with approximately 10-nm axial and lateral repeats. Differential length changes of the fibrils may generate a wide dynamic spectrum of helical and non-helical geometries allowing for directional motility in low Reynolds number environments. The presence of other cytoskeletal elements (FtsZ, FtsA, EF-TU, MreB) has been demonstrated only recently in Spiroplasma cells. The cellular and molecular structure and dynamics of spiroplasmas and their cytoskeletal elements are reviewed.

Mycoplasmas: a distinct cytoskeleton for wall-less bacteria

The bacterial genus Mycoplasma includes a large number of highly genomically-reduced species which in nature are associated with hosts either commensally or pathogenically. Several Mycoplasma species, including Mycoplasma pneumoniae, feature a multifunctional polar structure, the terminal organelle. Essential for colonization of the host and for gliding motility, the terminal organelle is associated with an internal cytoskeleton crucial to its assembly and function. This cytoskeleton is structurally and compositionally novel as compared with the cytoskeletons of other organisms, including other bacteria, is also involved in the cell division process. In this review we discuss the cytoskeletal structures and protein components of the attachment organelle and how they might interact and contribute to its various functions.

Physical and genetic map of the Spiroplasma kunkelii CR2-3x chromosome

Spiroplasma kunkelii (class Mollicutes) is the characteristically helical, wall-less bacterium that causes corn stunt disease. A combination of restriction enzyme analysis, pulsed-field gel electrophoresis (PFGE), and Southern hybridization analysis was used to construct a physical and genetic map of the S. kunkelii CR2-3x chromosome. The order of restriction fragments on the map was determined by analyses of reciprocal endonuclease double digests employing I-CeuI, AscI, ApaI, EagI, SmaI, BssHII, BglI, and SalI; adjacent fragments were identified on two-dimensional pulsed-field electrophoresis gels. The size of the chromosome was estimated at 1550 kb. Oligonucleotide pairs were designed to prime the amplification of 26 S. kunkelii gene sequences in the polymerase chain reaction (PCR). Using PCR amplicons as probes, the locations of 27 S. kunkelii putative single-copy genes were positioned on the map by Southern hybridization analyses of chromosomal fragments separated in PFGE. The nucleotide sequence of the single ribosomal RNA operon was determined and its location mapped to a chromosomal segment bearing recognition sites for SalI, SmaI, EagI, and I-CeuI.

The analysis of cell division and cell wall synthesis genes reveals mutationally inactivatedftsQandmraYin a protoplast-type L-form ofEscherichia coli

Cell division and cell wall synthesis are tightly linked cellular processes for bacterial growth. A protoplast-type L-form Escherichia coli, strain LW1655F+, indicated that bacteria can divide without assembling a cell wall. However, the molecular basis of its phenotype remained unknown. To establish a first phenotype-genotype correlation, we analyzed its dcw locus, and other genes involved in division of E. coli. The analysis revealed defective ftsQ and mraY genes, truncated by a nonsense and a frame-shift mutation, respectively. Missense mutations were determined in the ftsA and ftsW products yielding amino-acid replacements at conserved positions. FtsQ and MraY, obviously nonfunctional in the L-form, are essential for cell division and cell wall synthesis, respectively, in all bacteria with a peptidoglycan-based cell wall. LW1655F+ is able to survive their loss-of-functions. This points to compensatory mechanisms for cell division in the absence of murein sacculus formation. Hence, this L-form represents an interesting model to investigate the plasticity of cell division in E. coli, and to demonstrate how concepts fundamental for bacterial life can be bypassed.

Borrelia burgdorferi ftsZ plays a role in cell division

ftsZ is essential for cell division in many microorganisms. In Escherichia coli and Bacillus subtilis, FtsZ plays a role in ring formation at the leading edge of the cell division septum. An ftsZ homologue is present in the Borrelia burgdorferi genome (ftsZ(Bbu)). Its gene product (FtsZ(Bbu)) is strongly homologous to other bacterial FtsZ proteins, but its function has not been established. Because loss-of-function mutants of ftsZ(Bbu) might be lethal, the tetR/tetO system was adapted for regulated control of this gene in B. burgdorferi. Sixty-two nucleotides of an ftsZ(Bbu) antisense DNA sequence under the control of a tetracycline-responsive modified hybrid borrelial promoter were cloned into pKFSS1. This construct was electroporated into a B. burgdorferi host strain carrying a chromosomally located tetR under the control of the B. burgdorferi flaB promoter. After induction by anhydrotetracycline, expression of antisense ftsZ RNA resulted in generation of filamentous B. burgdorferi that were unable to divide and grew more slowly than uninduced cells. To determine whether FtsZ(Bbu) could interfere with the function of E. coli FtsZ, ftsZ(Bbu) was amplified from chromosomal DNA and placed under the control of the tetracycline-regulated hybrid promoter. After introduction of the construct into E. coli and induction with anhydrotetracycline, overexpression of ftsZ(Bbu) generated a filamentous phenotype. This suggested interference of ftsZ(Bbu) with E. coli FtsZ function and confirmed the role of ftsZ(Bbu) in cell division. This is the first report of the generation of a B. burgdorferi conditional lethal mutant equivalent by tetracycline-controlled expression of antisense RNA.

Phytoplasmas and their interactions with hosts

Phytoplasmas are bacteria without cell walls and are responsible for plant diseases that have large economic impacts. Knowledge of their biology is limited because they are uncultivable and experimentally inaccessible in their hosts. It is a mystery how these bacteria use the sugar-rich phloem sap in which they live and how they interact with the host. This makes it difficult to develop means to control them. Recently, the full genomes of two phytoplasmas have been sequenced, allowing new insights into their requirements. Phytoplasmas contain a minimal genome and lack genes coding for ATP synthases and sugar uptake and use, making them dependent on their host. This dependency can be exploited to elucidate the particular physiology of the phloem.