MOVEMENTS - Cyanobacteria - page 297

chromosome as pix L-N and pix L-C . (Yoshihara et al ., 2001). PatA is a cyanobacteria-specifi c response
regulator which contains a region similar to CheY of E . coli that was reported in Anabaena sp. strain
7120, known to control heterocyst pattern formation (Liang et al ., 1992). CheY homologue interacts
with either pilT or pilU proteins that supply energy for retraction (Yoshihara et al ., 2000; Bhaya et
al ., 2001).
On the other hand, the photoreceptor protein for phototaxis in S . elongatus is localized at the
two poles of the cells. This photoreceptor protein is a product of the gene SepixJ (a constituent of
Synechococcus elongatus pix -gene cluster) that is homologous to pixJ of Synechocystis sp. strain PCC
6803. Furthermore, the SepixJ is an important constituent of the Sepix G gene cluster having SepixGHIJL
that is homologous to pix G gene cluster ( pixGHIJ1J2L ) of Synechocystis sp. strain PCC 6803 (Kondou
et al ., 2002). Ishizuka et al. (2006) studied the characteristics of PixJ of T . elongatus BP-1 and this CBCR
exhibited photoconversion between blue light (433 nm) and green light (531 nm) absorbing forms.
On the basis of its expression studies in Synechocystis sp. strain PCC 6803 it was concluded that PVB
acts as the chromophore in contrast to PCB of cph1 .
Okajima et al . (2005) suggested that the phytochrome-like photoreceptor and BLUF act as
master switch between positive and negative phototaxis. If in fact BLUF is the photoreceptor, then it
should be able to interact with PatA-like response regulator. Since sequence analysis of the genome
of Synechocystis sp. strain PCC 6803 revealed six homologous patA -like genes (Kaneko et al ., 1996),
Okajima et al. (2005) conducted yeast two-hybrid screening that confi rmed the product of pixD
(i.e. BLUF) self-interacted and interacted with pixE proteins strongly suggesting the existence of a
possible signal transduction pathway. They speculated that the blue-light photoreceptors might be
active for switching on negative phototaxis to avoid photoinhibition although photosynthetic light is
available. It is clear that there are many gaps in understanding the mechanism of phototaxis. Though
the structures associated with gliding, i.e. fi brillar apparatus, JPC and the spicules are described,
how these perform their functions is yet to be identifi ed. It is not enough to know the presence of
gene sequences homologous to known sequences found in Synechocystis sp. strain PCC 6803. For
example, genes essential for motility and positive phototaxis of Synechocystis sp. strain PCC 6803
have been found in Anabaena sp. strain PCC 7120 (Kaneko et al ., 2001) and Nostoc punctiforme (Meeks
et al ., 2002). However, the former does not develop harmogonia while the latter forms hormogonia
which exhibit gliding. Further, it is of interest to know that the motile harmogonia produced by
Calothrix sp. PCC 7601 differentiates pili-like structures (Damerval et al ., 1991). The role of such pili
in gliding is yet to be worked out.
LITERATURE CITED
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Adams, D. G., Ashworth, D., and Nelmes, B. (1999) Fibrillar array in the cell wall of a gliding fi lamentous cyanobacterium.
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Alexandre, G., and Zhulin, I. B. (2001) More than one way to sense chemicals. J Bacteriol 18: 4681 - 4686.
Armitage, J. P., and Hellingwerf, K. J. (2003) Light-induced behavioral responses ('phototaxis') in prokaryotes. Photosynth
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Bardy, S. L., Ng, Y. M. S., and Jarrell, K. F. (2003) Prokaryotic motility structures. Microbiology 149: 295 - 304.
Bhaya, D. (2004) Light matters: phototaxis and signal transduction in unicellular cyanobacteria. Mol Microbiol 53: 745-754.
doi:10.1111/j.1365-2598.2004.04160.x
Bhaya, D., Bianco, N.R., Bryant, D., and Grossman, A. (2000) Type IV pilus biogenesis and motility in the cyanobacterium
Synechocystis sp. PCC 6803. Mol Microbiol 37: 941 - 951.
Bhaya, D., Nakasugi, K., Fazeli, F., and Burriesci, M. S. (2006) Phototaxis and impaired motility of Synechocystis sp. strain
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