MOVEMENTS - Cyanobacteria - page 282

and adventurous gliding. Social gliding is exhibited by groups of cells and adventurous gliding by
single cells. It is interesting to note that social gliding is mediated by Tfp. Besides helically arranged
protein fi brils, JPC is another organelle that helps M . xanthus in its adventurous gliding (Mattick,
2002; Nudleman and Kaiser, 2004).
A number of eubacteria and certain fi lamentous cyanobacteria exhibit surface-linked movements
known as gliding movements (Castenholz, 1973, 1982; Hoiczyk, 2000; McBride, 2000, 2001; Adams,
2001). In the latter, helically arranged protein fi brils and JPC help in the gliding process by secreting
mucilage (Wolgemuth et al ., 2002; Wolgemuth and Oster, 2004). Ratchet structure is a characteristic
feature of Cytophaga-Flavobacterium group of bacteria that show gliding due to the movement of cell
surface components. In these organisms, specifi c motility proteins are anchored to the cytoplasmic
and outer membrane. Movements of the cytoplasmic proteins may be driven by the proton motive
force and their interaction with the outer membrane proteins may propel the cells forward (Bardy
et al ., 2003). Contractile cytoskeleton is a peculiar structure seen in Spiroplasma melliferum which
exhibits motility in the absence of fl agella and genes that regulate gliding. The cytoskeleton consists
of an unique 59 kDa protein which is supposed to act as the linear motor that is internally attached
to the cytoplasmic membrane.
Movements in response to light are classifi ed into phototaxis, photokinesis and photophobic
responses. Phototaxis is described as movement directed to the orientation of incident light while
photokinesis is the speed of movement regulated by total light intensity. Photophobic response is
the reversal of the direction of movement (Diehn et al ., 1979). Likewise, movements in response to
chemical environment are known as chemotactic movements while those in relation to water are
termed as hydrotactic movements. How do cyanobacteria sense and respond to light, their phototactic
behaviour and how does light play a role in Tfp-dependent motility are the areas which received
attention during the past two decades (Häder, 1987a,b; Bhaya et al ., 2001; Mullineaux, 2001; Armitage
and Hellingwerf, 2003; Nakasugi et al ., 2006, 2007).
Bacterial chemotaxis has been investigated in very great detail with Escherichia coli emerging as
the best example. There are three basic steps in bacterial chemotaxis. The fi rst is signal perception
by bacterial chemoreceptors located in the membrane. The second is signal transduction from
chemoreceptors to the motor of the fl agella and the third is the adaptation of the signal to desensitize
the initial signal output (Lux and Shi, 2004). A chemoreceptor is a transmembrane protein that is
divided into three portions, i.e. an extra-cytoplasmic ligand-binding portion and a transmembrane
portion that connects to a cytoplasmic portion consisting of the signalling and methyl-accepting
domains. That is why these are called as methyl-accepting chemoreceptor proteins (MCPs) or
methyl-accepting chemotaxis proteins (Kort et al ., 1975). Located at the poles of the bacterial cell,
MCPs are constitutive and are not involved in transport or metabolism. These are very sensitive
detection devices that can detect concentrations of specifi c ligands at concentrations ranging from
µM to nM range (Clarke and Koshland, 1979; Biemann and Koshland, 1994; Lin et al ., 1994). This is
possible because the MCPs function as signalling lattices by forming initially stable homodimers
and later these tend to become trimers of dimers (Lux and Shi, 2004). Signals perceived by MCPs
are transmitted across the membrane which triggers an excitation response that is conveyed to two
sensory proteins, i.e. CheA (a histidine kinase) and CheY (a response regulator). CheA interacts
with the cytoplasmic signalling domain of MCP in association with CheW. As a result of which a
tight ternary complex is produced. CheY, a response regulator phosphorylated by CheA, can bind
to the motor of the fl agella and affect a change in the direction of rotation and thus in the swimming
direction (Armitage and Hellingwerf, 2003; Lux and Shi, 2004). These transmembrane chemoreceptors
constitute nearly 90% of the total number of chemotaxis transducer molecues in E. coli (Hazelbauer