MOVEMENTS - Cyanobacteria - page 289

(Lounatmaa et al ., 1980; Dick and Stewart, 1980; Vaara, 1982; Vaara et al ., 1984; Vaara and Vaara,
1988). But their formation, function and mechanisms of cellular and environmental controls have
not been clearly understood. That is why a few of the cyanobacteria that exhibit gliding motility
have been re-examined for the presence of fl agella or pili. Amongst these, P. uncinatum and the
marine Synechococcus species did not reveal any such appendages (Waterbury et al ., 1985: Hoiczyk
and Baumeister, 1995, 1997).
Synechocystis sp. strain PCC 6803 possesses peritrichous pili of two distinct types, i.e. thick pili,
(i.e. Tfp) having a diameter of 6-8 A° and a variable length of 2.5 µm, are required for twitching and
thin pili (with a diameter of 3-4 A°), shorter than Tfp and covering the entire cell surface, are not
required for motility (Bhaya et al ., 1999; Fig. 3a,b). The twitching motility of Synechocystis sp. strain
PCC 6803 is akin to the social gliding shown by M. xanthus . When plated on agar plates, Synechocystis
sp. PCC 6803 forms irregular, fl at and spreading colonies and the cells exhibit phototactic movement
en masse, whereas the mutant cells of this organism form dome-shaped and circular colonies that
remain at the same place where they are seeded.
Tfp are composed of pilin sub-units (Sastry et al ., 1983). Each sub-unit is synthesized as a
precursor which undergoes cleavage by a specifi c pre-pilin peptidase that cuts off the N-terminal
part. This part of the mature pilin is highly conserved while the C-terminal part is not (Sastry et al .,
1985). Although the arrangement of pilin sub-units of Tfp in Synechocystis sp. strain PCC 6803 is
not known, in N . gonorrhoeae it is composed of fi ve pilin monomers per helical turn with a rise of
approximately 41 Å per turn (Mattick, 2002). Calcium-dependent motility of Synechocystis sp. strain
PCC 6803 was visualized by computer-aided video-microscopy (Moon et al ., 2004). On the basis
of chelation studies and the use of calcium channel inhibitor (pimozide), calcium effl ux blocker
(orthovanadate), calcium ionophore (A23187) and calmodulin antagonists (trifl uoroperazine and
chlorpromazine), they concluded that Ca 2+ plays a signifi cant role in regulating photomovement.
Twitching movements take place in short intermittent jerks of up to several micrometers. The
cells require a very moist surface and also cells have to exist in groups. Twitching is due to the
presence of Tfp. The retraction of the pili provides the necessary driving force for twitching. Besides
twitching, Tfp also are helpful in transformation, infection and adhesion (McBride, 2000, 2001). At
least 12 Tfp components have been shown to affect transformation of Synechocystis sp. strain PCC
6803. Of these, PilA1 and PilT1 exert profound infl uence since pilA1 mutants are defi cient in Tfp on
their cell surface while those of pilT1 are hyperpiliated (Yoshihara et al ., 2001; Okamoto and Ohmori,
2002; Nakasugi and Neilan, 2005; Nakasugi et al ., 2006). Gene slr0388 of Synechocystis sp. strain PCC
6803, annotated earlier under hypothetical proteins, has now been identifi ed as a comF gene that
regulates natural transformation, phototactic motility and piliation. This constitutes the second such
gene that regulates transformation competency in Synechocystis sp. strain PCC 6803, the fi rst one
being comA (Yoshihara et al ., 2001). Gene disruption mutants of comF synthesized abundant PilA1
protein while at the same time the transcript and corresponding protein levels of PilT1 and ComA
remained unaffected. The mutants exhibited a clumping phenotype and lost phototactic motility as
well as transformation potential (Nakasugi et al ., 2006). However, the essentiality of another gene
( ssr3341 ) for motility and natural transforming ability of Synechocystis sp. stain PCC 6803 has been
demonstrated. This gene encodes an RNA chaperone (Hfq) which is reported to have multiregulatory
roles in improving functional effi ciency and stability of a number of small regulatory RNAs and
overall translational performance. Inactivation of this gene caused loss of both phototactic and
transformation abilities of the mutant and DNA microarray analysis of this mutant showed that
the expression of a set of genes ( slr1667 , slr1668 , slr2015 , slr2016 and slr2018 ), involved in motility,