MOVEMENTS - Cyanobacteria - page 294

action peaks (at 720 and 740 nm) appeared. The motility rates of the cells were found to be 0.1 to
0.3 µm s -1 . However, in Synechocystis sp. strain PCC 6803 the absorption spectrum for positive
phototaxis has been reported to be between 560 to 720 nm (Choi et al. , 1999; Ng et al ., 2003). Likewise,
the action spectra for negative phototaxis of Synechocystis sp. strain PCC 6803 has been found to be
in the longer ultraviolet (360 nm), high intensity blue light (470 nm) and red light regions (600 to
700 nm) while in A . variabilis it lies between 500-560 nm and 700nm ( Nultsch et al., 1983 ; Choi et al .,
1999). The infl uence of light and temperature on the phototaxis of Synechococcus isolates from hot
springs of Octopus Yellow Stone National Park was studied (Ramsing et al ., 1997). The involvement
of a phytochrome-like receptor in the phototaxis of Synechocystis sp. strain PCC 6803 was predicted
based on the wave length dependency and phototactic speed (Choi et al ., 1999).
There appears to be a great diversity in the type and functional role of the photoreceptor
pigments in cyanobacteria. The fi rst phytochrome-like protein was discovered in Fremyella
diplosiphon that regulates complementary chromatic adaptation (Kehoe and Grossman, 1996).
Cyanobacterial phytochromes (Cph1 and Cph2) are akin to plant-type phytochromes and have
been characterized in Synechocystis sp. strain PCC 6803. The chromophore in plant phytochromes
is known as phytochromobilin wheras in Cph1 and Cph2 the chromophore is phycocyanobilin. In
general, the N-terminal part of the protein consists of the photosensory core and the C-terminal
region consists of a histidine kinase that is regulatory in nature. GAF domain is the photosensory
domain in most of the phytochromes and phytochrome-related proteins such as Cph1 and Cph2
(Rockwell et al ., 2006). Cph1 is a light-regulated biliprotein kinase and the cph1 gene is located
upstream of a gene that encodes a response regulator to Cph1 in the genome of Synechocystis sp.
strain PCC 6803. Typical of the plant-type phytochromes, Cph1 undergoes the classical red [P(r)] and
far-red light [P(fr)] shifts and these chromophore structures are well conserved (Hughes et al ., 1997;
Yeh et al ., 1997). Cph2 of Synechocystis sp. strain PCC 6803 differs from Cph1 in lacking the histidine
kinase domain (Park et al ., 2000) but contains GGDEF and EAL output domains (Montgomery
and Lagarias, 2002). Rockwell et al . (2009) identifi ed that the P(fr) states of bacteriophytochromes
and Cph1 are structurally dissimilar and it is the propionate side chains that perform different
functions in these two classes. The diversity of photoreceptor pigments in cyanobacteria is known
collectively as cyanobacteriochromes (CBCRs) that have absorption maxima spanning the entire
visible spectrum (Rockwell et al ., 2010). Though the CBCRs differ in their spectral properties they
share a common mechanism of light sensing by the 15/16 double-bond of the bilin chromophore.
One of the subfamilies of CBCRs is distinctive in showing blue/green light sensing and these have
evolved a thioether linkage to a second cysteine within the bilin-binding GAF domain (Rockwell et
al ., 2011). They have in addition characterized the dual cysteine photosensors from N . punctiforme
ATCC 29133. Mutants of Synechocystis sp. strain PCC 6803, isolated after site-directed mutagenesis
of Cph1, acquired the potential to sense light from near UV light to infrared range (Rockwell et al .,
2011). Furthermore, it has been demonstrated that the second cysteine residue in the bilin-binding
GAF domain is well conserved in the DXCF motif (Asp-Xaa-Cys-Phe). Such CBCRs perform blue/
green light shifts by exhibiting isomerization of phycocyanobilin (PCB) into phycoviolobin (PVB),
respectively. More than ten phycoviolobin variants of the CBCR subfamily from N . punctiforme
ATCC 29133 have been produced in E . coli by genetic engineering techniques and the recombinant
proteins exhibited well resolved blue/green light sensing states (Rockwell et al ., 2012). Moon et al
(2010) reported the role of cyanopterin in UV/blue light signal transduction of Synechocystis sp.
strain PCC 6803 phototaxis. Based on the properties of pterine glycosyltransferase gene ( pgtA )-
inactivated mutants of Synechocystis sp. PCC 6803 and inhibitory studies on pterin biosynthesis in