The bacterial cell division protein FtsZ from Escherichia coli has been purified with a new calcium precipitation method. The protein contains one GDP and one Mg 2؉ bound, it shows GTPase activity, and requires GTP and Mg 2؉ to polymerize into long thin filaments at pH 6.5. FtsZ, with moderate ionic strength and low Mg 2؉ concentrations, at pH 7.5, is a compact and globular monomer. Mg 2؉ induces FtsZ self-association into oligomers, which has been studied by sedimentation equilibrium over a wide range of Mg 2؉ and FtsZ concentrations. The oligomer formation mechanism is best described as an indefinite self-association, with binding of an additional Mg 2؉ for each FtsZ monomer added to the growing oligomer, and a slight gradual decrease of the affinity of addition of a protomer with increasing oligomer size. The sedimentation velocity of FtsZ oligomer populations is compatible with a linear singlestranded arrangement of FtsZ monomers and a spacing of 4 nm. It is proposed that these FtsZ oligomers and the polymers formed under assembly conditions share a similar axial interaction between monomers (like in the case of tubulin, the eukaryotic homolog of FtsZ). Similar mechanisms may apply to FtsZ assembly in vivo, but additional factors, such as macromolecular crowding, nucleoid occlusion, or specific interactions with other cellular components active in septation have to be invoked to explain FtsZ assembly into a division ring.The FtsZ protein, which is found in all known eubacteria and archaea as well as in chloroplasts with the exception of the Chlamydiae (1, 2), plays a central role in bacterial cell division (3-5). During the life of a cell, FtsZ molecules remain in the cytoplasm until the time previous to division when they localize at the midcell point, forming a ring at the inner face of the cytoplasmic membrane where the septum will form (6, 7). The interactions of FtsZ with other septator proteins that are functionally relevant, such as PBP3 and FtsA, are being discovered (reviewed in Refs. 4,5,8,and 9). Several regulators of FtsZ ring formation have been found that either prevent/inhibit polymer formation (like the SOS response protein SulA (10), the MinCDE complex (11), and more recently, the ErzA protein from Bacillus subtilis (12)) or stabilize the FtsZ polymer (like the membrane protein ZipA (13, 14)). Despite this information, FtsZ ring structure and assembly are still poorly understood.FtsZ binds guanine nucleotides and has GTPase activity (15-17), which is dependent on protein concentration and probably is linked to self-association equilibria (15,16,18). Multiple sequence alignment and secondary structure predictions indicate that bacterial FtsZ is a homolog of eukaryotic tubulins (19,20). The x-ray crystal structure of the FtsZ from the archaeon Methanococcus jannaschii (21) and the electron crystallographic model structure of the ␣␤-tubulin dimer (22) have similar folds.The nature of the polymers that FtsZ forms in vivo is unknown. FtsZ self-assembles in vitro to form filaments and rings (23-28...

The transcriptional response of Streptococcus pneumoniae was examined after exposure to the GyrB-inhibitor novobiocin. Topoisomer distributions of an internal plasmid confirmed DNA relaxation and recovery of the native level of supercoiling at low novobiocin concentrations. This was due to the up-regulation of DNA gyrase and the down-regulation of topoisomerases I and IV. In addition, >13% of the genome exhibited relaxation-dependent transcription. The majority of the responsive genes (>68%) fell into 15 physical clusters (14.6–85.6 kb) that underwent coordinated regulation, independently of operon organization. These genomic clusters correlated with AT content and codon composition, showing the chromosome to be organized into topology-reacting gene clusters that respond to DNA supercoiling. In particular, down-regulated clusters were flanked by 11–40 kb AT-rich zones that might have a putative structural function. This is the first case where genes responding to changes in the level of supercoiling in a coordinated manner were found organized as functional clusters. Such an organization revealed DNA supercoiling as a general feature that controls gene expression superimposed on other kinds of more specific regulatory mechanisms.