Shaping and Moving a <i>Spiroplasma</i>

Abstract

The Mollicutes (Spiroplasma, Mycoplasma and Acholeplasma) are the most minimal cells known to exist, being the smallest and simplest free-living and self-replicating forms of life. Phylogenetically, the Mollicutes are related to gram-positive bacteria and have evolved, by regressive evolution and genome reduction, from Clostridia. The smallest genome in this group (Mycoplasma genitalium – 5.77 × 10⁵ bp) is only twice that of a large virus (e.g., Entomopox viruses). The largest Mollicute genome (Spiroplasma LB12 – 2.2 × 10⁶ bp) is only about half that of, e.g., Escherichia coli. Structurally, the Mollicutes lack cell walls and flagella, but have internal cytoskeletons and are motile and chemotactic. Only a cholesterol-containing unit membrane envelops the cells. No analogs to the bacterial chemotactic and motility (che, mot, fla) genes, genes for a two-component signal transduction system, genes associated with gliding, or genomic homologs for the eukaryotic cytoskeleton and motor proteins were found in the Mollicutes. The Spiroplasmas are unique amongst the Mollicutes in having a well-defined basic helical cell geometry. In this respect, the Spiroplasma cell can, essentially, be viewed as a helical dynamic membranal tube (diameter ∼0.2 µm; equivalent to that of one eukaryotic flagellar axoneme or to a bacterial flagellar bundle). A flat cytoskeletal ribbon of parallel fibrils is attached to the inside of the cellular tube. Both tube and cytoskeleton are mutually coiled into a dynamic helix driven by differential length changes of the fibrils, which function as linear motors. The cytoskeletal ribbon follows the shortest (inner) helical line on the inner surface of the cellular tube. Being helical allows for further analytical reduction and consequent structural quantification of Spiroplasma. Of particular importance is the ability to correlate light and electron microscopy data and to calculate the fibril lengths (and corresponding molecular dimensions) in the helical and nonhelical dynamic states. The structural unit of the contractile cytoskeleton is a ∼50-Ångstrom-wide filament comprised of pairs of the 59-kD fib gene product. The monomers are arranged in pairs with opposite polarities allowing for a ∼100-Ångstrom-long axial repeat. The functional unit of the contractile cytoskeletal ribbon is a fibril comprised of an aligned pair of filaments. Neighboring repeats form a tetrameric ring with a lateral repeat of ∼100 Å. The axial length of the rings may shorten by ∼40%, driving the changes in the fibril lengths and, consequently, helical dynamics. Local length changes result in helical symmetry breaking and nonreciprocating cell movements allowing for net directional displacement. Flexing allows for changes in swimming direction.

Cytoskeleton of <i>Mollicutes</i>

“…A flat cytoskeletal ribbon of parallel fibrils is attached to the cellular tube and follows the shortest helical line on the inner surface of the cellular tube. Contraction of the fibrils may cause the changes in the handedness of helical cells and the resulting kinks may propel the cell, as discussed elsewhere [Berg, 2002;Kurner et al, 2005;Shaevitz et al, 2005;Trachtenberg, 1998Trachtenberg, , 2004.…”

The Flagellar Cytoskeleton of the Spirochetes

“…Interestingly, Spiroplasma , a swimming wall-less bacterium, can swim faster in non-gel-like high-viscosity media, which suggests that its swimming is driven by a different mechanism [Trachtenberg, 2004;Wolgemuth et al, 2003]. The PFs of spirochetes are known to be rather similar in structure to bacterial flagella [Limberger, 2004], with a flexible hook region in the basal body that connects to a helical flagellar filament.…”

The Monomeric, Tetrameric, and Fibrillar Organization of Fib: The Dynamic Building Block of the Bacterial Linear Motor of Spiroplasma melliferum BC3

“…9,10 The propellers are extracellular or rotate within the periplasmic space. 11,12 Spiroplasmas are the only bacteria known to swim by means of a contractile linear motor [13][14][15] rather than a rotary one. Spiroplasmas belong to the class Mollicutes (which include also mycoplasmas and acholeplasmas 16 ), representing the minimal, freeliving, and self-replicating forms of life.…”