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.