MOVEMENTS - Cyanobacteria - page 286

(P-11-16 and P-11-17) (Perkins et al ., 1981) and a freshwater strain of Synechococcus (Schultze-Lam
and Beveridge, 1994) which lack motility.
It is signifi cant to note that Samuel et al . (2001) employed cryopreservation and freeze-substitution
techniques to study the envelope structure of Synechococcus sp. strain WH8113. The ultrastructure
studies of the cell envelope revealed an S-layer (of approximately 35 nm thick), outer membrane (10
nm thick), a peptidoglycan layer (15 nm thick) and cell membrane (10 nm thick). The presence of
profusely large number of spicules (as long as 150 nm at a distance of 12 to 24 nm), with a uniform
thickness of 5 nm, have been demonstrated by them (Fig. 1a,b). The radius of the spicules is found to
be approximately 30 nm. The spicules are suggested to extend through channels in the surface layer
and pass through the outer membrane, peptidoglycan layer and the cell membrane. The deprivation
of calcium ions caused a disruption not only in the integrity of the spicules but also resulted in a
separation of the thylakoid membrane (Fig. 2). The structural arrangement of spicules has been
correlated to their function. Thus Synechococcus sp. strain WH8113 could swim by oscillating its
surface (obviously by the rowing motion caused by the spicules) in the form of a travelling wave.
The power for motility is derived from the ion-motive force across the cell membrane as suggested
by Willey et al . (1987).
The genome of marine Synechococcus sp. strain WH8102 has been sequenced and analysis of
the 2.4 mega base genome revealed three major portions, i.e. (i) a highly conserved core (consisting
of nearly half the genome) associated with regions or gene sequences of marine adaptations (i.e.
Ni-dependent superoxide dismutase, carboxysomes and DNA repair); (ii) marine adaptations (Na-
dependent transporters) and (iii) a portion of genome unique to strain WH8102 consisting of gene
sequences governing motility, phage-related regions, effl ux transporters, glycosyltransferases, nitrate
reduction, phycobilisomes and carbonic anhydrase (Palenik et al ., 2003). Although Synechococcus sp
strain WH8102 does not possess pili or surface associated twitching motility, it is interesting to note
that the genome contains six pil-like genes. Of these at least pilB , pilC and pilD have been identifi ed.
However, these do not encode the full complement of proteins required for pilus assembly and
function (Palenik et al ., 2003). Genes required for motility are found in at least two widely separated
regions, i.e. Swm A and SwmB . Of these two, the latter is found to be a very large ORF for 10,791 amino
acids that is the longest ORF ever reported and constitutes more than 1% of the genome size.
i) Mechanism of swimming : There are two important steps in swimming. First is generation of thrust
and second is generation of torque. SwmA is shown to be required for the generation of thrust but
not torque (Brahamsha, 1996). The coupling of the generation of thrust and torque is required for
swimming but torque generation still remains unanswered. Currently two models are in vogue
to describe the mechanism of swimming. These are (a) self-electrophoresis model and (b) wave
generation model (Brahamsha, 1999).
a) Self-electrophoresis model : The cells are supposed to carry a fi xed charge on their surface that is
shielded by counter ions in the medium. If the cells are able to pump in ions at one end and pump
them out at the other end, an electric force will be set up in the external medium. The external fl uid
layer containing the counter ions are driven over the surface of cells just like the treads of a tank
that enables the cell to move in the opposite direction to the movement of external layer. Pitta and
Berg (1995) ruled out such a mechanism as the electrophoretic mobility of Synechococcus cells is
essentially zero.
b) Wave generation model : Ehlers et al . (1996) proposed this model by envisaging the generation of
longitudinal or transverse waves of 0.2 µm long, 0.02 µm of amplitude and travelling at 160 µm s -1