MOVEMENTS - Cyanobacteria - page 296

positive phototaxis is the typical action maxima for phytochrome mediated response in plants. Ng
et al . (2003) further emphasized that TaxD1 protein is an atypical phytochrome or the features and
activities of the phytochrome-like receptor are modifi ed by the association of TaxD1 in vivo .
Spectroscopic analysis of heterologously expressed Synechocystis PixD exhibited blue/green
light shifted forms characteristic of fl avin-bound BLUF domain (Masuda et al., 2004). A new
dimension to the regulation of phototaxis was added by the studies of Okajima et al . (2005) who
demonstrated that there can exist two or more photoreceptors (other than phytochrome-like
receptor) that mediate phototaxis in cyanobacteria. A comparative study of Synechocystis sp. strain
PCC 6803 and S . elongatus for a signalling mechanism that regulates phototaxis revealed BLUF
(a sensor of Blue-Light Using FAD) as the photorceptor protein. While phytochrome-like
photoreceptor and BLUF perceive blue light, they even predicted that the existence of a probable
photoreceptor that is active in orange-red light. These studies are consistent with the action spectra
for phototaxis because monochromatic light of 500-700 nm was effective while light below 500 nm
was ineffective. BLUF is the gene product of pixD . Gene disruption studies invoving pixJ1 and pixD
showed that the mutants exhibited negative phototaxis toward monochromatic light of 500-700 nm.
The crystal structure of BLUF protein of T . elongatus BP-1 revealed it to be a decameric assembly
and the monomers (encoded by Tll0078 ) consist of a BLUF domain at its N-terminal region. The
overall structure of the BLUF domain consisted of fi ve-stranded β-sheet with two α-helices running
parallel with it (Kita et al ., 2005). The intramolecular hydrogen bonded FAD chromophore exhibited
photochemical reaction and the role of conserved Gln50 and Tyr8 has been detected to be critical
(Kita et al ., 2005; Okajima et al ., 2006). The structure of Synechocystis sp. strain PCC 6803 PixD (a
product of slr1694 ) has been subsequently reported by Yuan et al . (2006) where the BLUF domain of
Synechocystis resembled the BLUF domain of Tll0078 of T . elongatus BP-1. The role of the conserved
Gln50 and Tyr8 has been confi rmed by them in Synechocystis sp. strain PCC 6803 as reported earlier by
Okajima et al . (2006). Additionally, the role of Trp91 in interacting with fl avin to bring conformational
changes in response to light absorption has been highlighted (Yuan et al ., 2006). Masuda et al . (2008)
identifi ed the crucial role of the conserved Met93 in the BLUF protein (PixD/ slr1694 ) of Synechocystis
sp. strain PCC 6803 for light-dependent signal transduction and suggested that the Met93 residue
is highly conserved in all BLUF domains. Fukushima et al . (2008) studied the properties of mutants
of T . elongatus BP-1 where the conserved amino acid residues Tyr8 and Gln50 of the BLUF protein
were modifi ed to Phe, Ala and Asn (Tyr8Phe, Tyr8Ala, Gln50Asn and Gln50Ala) by site-directed
mutagenesis. All the mutants lacked the typical response of red shift upon blue-light illumination
exhibited by the wild-type. Tanaka et al . (2009) reported oligomeric-state-dependent conformational
changes of BLUF protein of T. elongatus (TePixD/Tll0078) and the protein existed in pentameric and
decameric states and it is the latter that brought about the observed conformational change in the
BLUF protein and so is functionally important.
B) Pix-genes and mechanism of phototaxis
Signal transduction pathway for phototactic motility in Synechocystis sp. strain PCC 6803 has been
predicted to be on similar lines as that establihed for bacterial chemotaxis. Accordingly, the gene
cluster governing positive phototaxis pixGHIJ1J2L releases products similar to PatA, CheY, CheW,
MCPs and CheA, respectively (Yoshihara et al ., 2000; Bhaya et al ., 2001). Thus it was suggested
that the gene cluster pixGHIJ1J2L forms the regulatory machinery for light perception and signal
transduction pathway for phototactic movement in Synechocystis sp. strain PCC 6803. The gene for
CheA in Synechocystis sp. strain PCC 6803 is present as a split gene at separate loci on the bacterial