MOVEMENTS - Cyanobacteria - page 285

a fi lament? (ii) secretion of mucilage by JPCs is so forceful enough that it can provide suffi cient
force to move a fi lament? and (iii) slime secretion is the basis for movement or is it extruded as a
consequence of motility? However, most of the corollary evidences point towards the following: (i)
because of slime extrusion the gliding motion occurs as supported by the relationship between the
speed at which the secretion of mucilage in Synechococ cus takes place; (ii) the direction of mucilage
secretion and locomotion are opposite to each other and (iii) mucilage secretion is able to provide
necessary power for gliding. The mechanism of gliding by bacterial and cyanobacterial cells by
slime extrusion model was explained by Jeon and Dobrynin (2005) who performed molecular
dynamics simulations by compressing polymer chains through a molecular nozzle. The force with
which the polymer chains are extruded through the molecular nozzle provides the force suffi cient
for propulsion.
B) Swimming
Waterbury et al . (1985) discovered that a marine species of Synechococcus sp. strain WH8102 could
swim in the absence of any specifi c organs of motility unlike other eubacteria which are able to swim
by means of fl agella. The swimming speeds ranged from 5 to 25 µm s -1 (Waterbury et al ., 1985; Willey,
1988). During swimming the cells rotate about their longitudinal axis as they move and so generate both
torque and thrust. Willey (1988) further observed that the swimming activity slows down and the cells
become immotile due to the increase in the viscosity of medium. A number of techniques (transmission
electron microscopy, freeze-fracture, freeze-etching, high intensity dark-fi eld microscopy and motility-
dependent amplitude spectra) employed were not helpful in unraveling any of the structures associated
with swimming in Synechococcus sp. strain WH8102 (Waterbury et al ., 1985; Willey, 1988).
The swimming motility has been attributed due to the presence of a glycoprotein situated on
the cell surface (constituting the S-layer) which is not an integral outer membrane protein but can
easily be detached from cells by treatment with EDTA. This glycoprotein, designated as SwmA, has
a molecular weight of 130 kDa (Brahamsha, 1996). SwmA is a glycosylated polypeptide consisting
of 835 amino acid residues and contains repeats of Gly and Asp that serve as calcium-binding
motifs (Brahamsha, 1999). There are a number of similarities between SwmA, oscillin and HylA
(Brahamsha, 1996) in having multiple repeats of calcium-binding motifs in the N-terminal region
and are homologous to the 47 amino acid domain at the C-terminal region without calcium-binding
motifs (Hoiczyk and Baumeister, 1997). A functional relationship between SwmA and oscillin has
also been indicated. While oscillin is shown to be associated with generating thrust for gliding with
the help of slime produced on solid surface, SwmA alters the cell's surface characteristics or shape in
such a way that rotation of cell results in thrust. The mechanism of swimming by Synechococcus is not
clearly understood due to lack of knowledge on: (1) the number of genes and their gene products,
(2) the presence of proteins other than SwmA and (3) the factors that generate torque and the signal
transduction pathway. A beginning in this direction has been made by the isolation of a mutant strain,
i.e. Synechococcus sp. strain S1A1, after insertional inactivation of swmA gene. It possesses major and
minor cell surface polypeptides found in the wild-type and is shown to be defi cient only in SwmA
protein due to which it lost motility along with the outer S-layer (McCarren et al ., 2005). In addition,
they noted fi brillar structures intervening the region between the S-layer and outer membrane. The
presence of S-layers is also reported in other gliding cyanobacteria as discussed above (Hoiczyk
and Baumeister, 1995; Hoiczyk and Hansel, 2000; Smarda et al ., 2002). The fi brillar structures that
characterize the space in between the S-layer and the outer membrane in Synechococcus sp. strain
WH8102 (McCarren et al ., 2005) also are reported to be present in marine Synechococcus isolates