MOVEMENTS - Cyanobacteria - page 293

(Yoshihara et al ., 2002). The primary motility genes are, i.e. pilA1 , pilB1 , pilC , pilD , pilM , pilN , pilO ,
pilQ and pilT which are also required for transformation competency (Bhaya et al ., 2000; Yoshihara
et al ., 2001). There are eleven pilA -like genes that encode pre-pilins. Thick and thin pilus biogenesis
is governed by pilA1 . PilA9 , pilA10 and pilA11 are responsible for cell motility. The other pilA -like
genes are not essential for motility or phototaxis (Yoshihara et al ., 2001). Except in case of pilT1 the
rest of the mutants completely lost the pili from their surface. Exceptionally pilT1 mutant exhibited
enhanced assembly of the pili (Okamoto and Ohmori, 1999; Bhaya et al ., 2000). The expression of
pilA , pilB , pilC and pilT genes of M. aeruginosa PCC 7806 has been studied and purifi ed PilT protein
showed ATPase activity. Heterologous expression of pilT gene, from M . aeruginosa PCC 7806, could
complement the pilT mutant phenotype of Pseudomonas aeruginosa but not that of Synechocystis sp.
strain PCC 6803 (Nakasugi et al ., 2007).
The secondary genes are identifi ed to be Ser/Thr protein phosphatase genes and Ser/Thr protein
kinase genes that are not linked to pilus assembly. Kamei et al . (1998) identifi ed a novel gene involved
in high-light resistance in the cyanobacterium Synechocystis sp. strain PCC 6803 and showed that
this gene sequence slr2031 plays a crucial role in the motility of this organism. A eukaryote-type
protein kinase designated as SpkA (Synechocystis protein kinase) gene is shown to regulate motility
in Synechocystis via phosphorylation of the membrane proteins. This is evidenced by the isolation
of the SpkA-disrupted mutant lacking motility (Kamei et al ., 2001). The molecular mechanism by
which the protein phosphorylation regulates motility in Synechocystis sp. strain PCC 6803 is not yet
clear. The third type of genes governing positive phototaxis include a gene cluster designated as pix
genes, i.e. pixGHIJ1J2L (detailed below in phototaxis; Yoshihara et al ., 2000; Bhaya et al ., 2001).
II. PHOTOTAXIS
Phototactic movements carried out by gliding, swimming or twitching are helpful to the cyanobacteria
in enabling them to absorb maximum light for effi cient photosynthesis. Positive phototaxis is directed
towards the source of illumination while negative phototaxis is elicited in the opposite direction
away from the incident light.
A) Action spectra and nature of photoreceptors
The action spectra for positive as well as negative phototaxis for a few of the cyanobacteria have been
determined. Early studies on the action spectra for phototaxis of some fi lamentous cyanobacteria
revealed that photosynthetic pigments were involved in the photorception (Nultsch, 1961; 1962;
Tyagi, 1976). However, inhibitors of the photosynthetic electron transport chain were not effective
to inhibit phototaxis (Tyagi, 1976). Since action spectrum for negative phototaxis in A . variabilis
was in far red light (730 nm), so it was concluded that it is not the photosynthetic pigments that
are the photoreceptors for phototaxis but some other photoreceptor must be involved (Nultsch
and Schuchart, 1985). Certain of the unicellular cyanobacteria such as S . elongatus and Synechocystis
sp. strain PCC 6803 also exhibit phototactic movements ( Stanier et al ., 1971; Castes et al ., 1986;
Ramsing et al ., 1997 and Choi et al ., 1999). The action spectra for positive phototaxis appeared to be
species specifi c as in P . uncinatum (390, 480, 560 nm), Cylindrospermum alatosporum (450 and 640 nm),
A . variabilis (550 and 730 nm) and in the thermophilic Thermosynechococcus elongatus (640 nm) light
of different wave lengths (mentioned in parentheses) is utilized (Hader, 1987a; Kondou et al ., 2001).
T. elongatus showed several action peaks at 530, 570, 640 and 680 nm at a fl uence rate of 10 µmol
m -2 s -1 but at higher fl uence rates the red action peaks (640 and 680 nm) disappeared and far-red