Lugol

Cytoskeletal elements in bacteria
Peter L Graumann
It has become clear recently that bacteria contain all of the cytoskeletal elements that are found in eukaryotic cells, demonstrating that the cytoskeleton has not been a eukaryotic invention, but evolved early in evolution. Several proteins that are involved in cell division, cell structure and DNA partitioning have been found to form highlydynamic ring structures or helical filaments underneath the cell membrane or throughout the length of the cell. These exciting findings indicate that several highly dynamic processes occur within prokaryotic cells, during growth or differentiation, that are vital for a wide range of cellular tasks.
Addresses Biochemie, Fachbereich Chemie, Hans-Meerwein-Straße, ¨ Philipps-Universitat Marburg, 35032Marburg, Germany e-mail: graumann@staff.uni-marburg.de

Current Opinion in Microbiology 2004, 7:565–571 This review comes from a themed issue on Growth and development Edited by Mike Tyers and Mark Buttner Available online 27th October 2004 1369-5274/$ – see front matter # 2004 Elsevier Ltd. All rights reserved. DOI 10.1016/j.mib.2004.10.010

and thus are predominantly close to the cell poles,whereas RNA polymerase is present on the nucleoids themselves [4,5]. This general separation of transcription and translation is a dynamic process that is dependent on active transcription [6]. Replication, however, occurs at a centrally located, stationary replication factory [7], such that the chromosome moves through the replisome during replication, and duplicated regions on the chromosome areactively separated towards opposite cell poles by an as yet unidentified motor [8]. The SMC (structural maintenance of chromosomes) chromosome condensation complex localizes to one or two discrete foci on the nucleoids, which is dependent on timing during the cell cycle [9–11]. Several essential histidine kinases localize to the cell poles at different times during the cell cycle in Caulobactercrescentus [12] and the chemotactic machinery is present at one or both cell poles in several bacteria [13]. In addition, although bacterial cells can be round, vibrio-shaped (crescents) or rod-shaped, all cells divide in the middle to create two similar-sized daughter cells, with the division machinery localizing to the middle of the cells [14] in the form of a constricting ring. How is thissubcellular organization achieved? At least in some cases, the clear answer is by way of cytoskeletal elements. In this review, I highlight recent findings on cytoskeletal elements and discuss their essential implications in cell structure, division and DNA segregation.

Abbreviations FRAP GFP IF fluorescence recovery after photobleaching green fluorescent protein intermediate filament

Prokaryotictubulin and its role in cell division
In contrast to eukaryotic cells, which use actin (and myosin) or other means, in the case of plants, for cell division, bacteria generally employ the tubulin ortholog FtsZ for this process [15]. FtsZ forms a ring structure in the middle of the cell (slightly off-centre in C. crescentus) (Figure 1a), and recruits all other known cell division ¨ proteins to thissite [14]. Jan Lowe’s group demonstrated that FtsZ is a tubulin ortholog [16], which in eukaryotic cells forms microtubules that provide cellular tracks for organelle transport and that form the mitotic spindle apparatus, among other functions. Like tubulin (a dimer of a- and b-tubulin), FtsZ polymerises into hollow protofilaments (which can be straight or curved) as a result of GTP binding (Figure2a). Upon filament formation, GTP appears to be rapidly hydrolysed into GDP and inorganic phosphate [17]. The number of filaments that comprise the Z ring is unclear but the ring has been shown recently to be highly dynamic. Using FRAP (fluorescence recovery after photobleaching), the Erickson group showed that the ring is completely remodelled within a timeframe of about 30 s (i.e. that polymerised...