MOVEMENTS - Cyanobacteria - page 284

12 nm. The angle between the fi brils and the long axis of the fi laments in all the above studies was
noted to be 25 to 30º. It is suggested that the fi brils help the fi laments in rotating them about their
long axis as they glide. The motor or driving force for gliding is presumed to be provided by the
angle of the fi brils and the rotation of the fi laments and the direction of rotation is characteristic of
the species.
The fi brils are made up of a glycoprotein known as oscillin consisting of 646 amino acid residues.
The N-terminal region (of 554 amino acid residues) comprises of 46 repeats of a calcium-binding
motif followed by a 92 residue C-terminal domain. They suggested that oscillin is essential for
gliding on the basis of helically arranged fi brils, an S-layer underlying the oscillin fi brils and the
secreted slime. The thrust is said to be generated by an interaction between the extruded slime, the
surface of the cell and the solid surface (Hoiczyk and Baumeister, 1995). However, it is pertinent
to point out that oscillin does not show any similarity with any other known motor proteins but
shows similarity with SwmA protein identifi ed on the surface of a swimming Synechococcus and
HylA, another glycoprotein identifi ed from non-motile fi lamentous cyanobacterium Anabaena sp.
strain PCC 7120 (Brahamsha, 1996).
b) Structure of JPCs : The existence of fi ne pores of 14-16 nm in diameter grouped in a circumference
around the septa of at least two dozen species of cyanobacteria has been reported earlier (Guglielmi
and Cohen-Bazire, 1982). Due to the fact that these pores did not traverse through the cyanobacterial
walls, no defi nite role in gliding could be ascribed to them. Hoiczyk and Baumeister (1998)
unequivocally demonstrated through light and electron microscopy that these pores are in fact part
of a larger structure designated as JPC. The JPC is an organelle having a diameter of 70-80 nm and
a length of 32 nm that spans the peptidiglycan and outer membrane. The JPC is cylindrical with a
central bulge of 14 nm in diameter and two pores of 8 nm in diameter on either side. The JPCs were
further shown to encircle the cell and are present near the septa in both Phormidium uncinatum and
Anabaena variabilis. However, in the former the pores are aligned in a single row while in the latter
several rows of pores are present on either side of the septum. Another important feature is the angle
with which the JPCs are inclined relative to the cell axis. It is suggested that 30-40° angle provides
the necessary direction to the slime extrusion which helps in propelling the cells forward.
iii) Mechanism of gliding : Two models were put forward to explain the mechanism of gilding. (a)
Surface waves : Halfen and Castenholz (1970, 1971) proposed that the driving force for gliding may
be provided by the rhythmic distortion of the protein fi brils present in the cell wall leading to the
formation of a wave over the surface of the fi lament that extends from one end to the other. The
waves transmitted through the outer membrane interact with the substratum and thus move the
fi lament forward, i.e. in the opposite direction. The propagation of the wave in the reverse direction
results in a movement to the backward direction.
b) Extrusion of slime : The JPCs steadily secrete mucilage which fl ows in tight contact with the surface
of the fi lament that makes it adhere to the surface. This adhesion and continuous secretion of mucilage
fi nally causes locomotion of the fi lament. The change in the direction of slime extrusion explains
the reversal in the direction of gliding. The helically arranged oscillin fi brils help in rotating the
fi lament and the direction of rotation is determined by the orientation of the fi brils. Hoiczyk and
Baumeister (1955, 1998) put forward a hypothesis that the surface striations formed by the protein
fi brils act as channels for the extruded slime to fl ow along the fi lament.
This model has raised more questions than has provided answers for gliding motion. The
following questions require consideration: whether (i) there is any synchronization in the JPCs of