The periseptal annulus: An organelle associated with cell division in Gram-negative bacteria

Abstract

Evidence is presented that the site of cell division in Salmonella typhimurium is flanked by two circumferential zones of cell envelope differentiation, the periseptal annuli, which separate the division site from the remainder of the cell envelope. Each annulus is composed of a continuous structure in which the membranous elements of the cell envelope are closely associated with, the murein cytoskeleton. The paired annuli appear early in the division process and the region between them defines a new cellular domain, the.periseptal compartment, within which the division septum is formed.The process of bacterial cell division occurs by ingrowth-of the cell envelope from a narrow circumferential zone at the midpoint of the cell. This leads to segregation of the cytoplasm into two compartments and finally to separation of the daughter cells. Ingrowth of the new septum at the proper location requires that the molecular organization of the division site differ from the organization of the envelope over-the remainder of the cell. The nature of this local differentiation and the cellular mechanisms that initiate and maintain it at the proper location are not known.In this paper we describe an organelle that is associated with early stages of the division process in Gram-negative bacteria and that is likely to play a role in these events. Fixation (1.5 hr) and subsequent washing and postfixation with 2.0% OS04, were carried out in the presence of the respective plasmolyzing solutions. In the experiment described in Fig. 3, the cells were subsequently washed twice more with distilled water and resuspended for 20 min in a saturated aqueous solution of thiocarbohydrazide (Sigma) to enhance contrast in the final sections; excess thiocarbohydrazide was removed by two washes with distilled water and the cells were again exposed to 2% OsO4 prior to embedding. Embedding, sectioning, and counterstaining with lead citrate and uranyl acetate were performed as described (2). Serial. sections approximately 60 nm thick were mounted on carbon-backed Formvar-coated slotted (2 X 1 mm) copper grids and examined in a Hitachi HUIIE electron microscope at 75 kV accelerating voltage. RESULTS The cell envelopes of Gram-negative bacteria contain two membranes which completely surround the cell. Between the inner (cytoplasmic) and outer membranes lies the continuous murein layer, a rigid crosslinked peptidoglycan that provides the only known cytoskeletal structure of these and other bacteria (3). During normal cell division the three layers invaginate coordinately to form the new septum (Fig. la). METHODSWhen cells are plasmolyzed by brief exposure to hypertonic solutions, the resulting decrease in cytoplasmic volume causes the inner membrane to shrink away from the rigid murein/ outer membrane layer. As originally shown by Bayer (4, 5), the plasmolysis procedure reveals the presence of small zones where the cytoplasmic membrane fails -to pull away from the murein/outer membrane layer. These sites of membrane adhesion are thought to re...

Native cellular architecture of Treponema denticola revealed by cryo-electron tomography

“…6 and S2; Movies S2 and S3). These structures may be related to the periseptal annulus observed prior to the separation of the cytoplasmic cylinders in E. coli and Salmonella typhimurium (Macalister et al, 1983;Cook et al, 1987). More recently, slower diffusion of periplasmic proteins was observed at the constriction site at a late stage of C. crescentus cell division (Judd et al, 2005); consistent with a the transient occurrence of periplasmic periseptal structures.…”

Envelope structure of four gliding filamentous cyanobacteria

“…1E and F). These circumferential junctions resemble the periseptal annuli described for several other eubacteria (32). The junctions separate the part of the periplasmic space between the outer membrane and the peptidoglycan of the single cells within the filaments.…”

Duplication and segregation of the actin (MreB) cytoskeleton during the prokaryotic cell cycle

“…Because the doublet is first visible as a pair of closely apposed individual rings that then move apart, it is possible that the MreB ''singlet'' ring may be a tightly packed doublet. The only other known paired structures that flank the division site are the continuous zones of adhesion (periseptal annuli) between inner membrane and the murein-outer membrane layer that have been reported to extend partially or completely around the cell cylinder on either side of the division site (24,25). It is not known whether there is a relationship between these structures and the MreB doublet elements.…”