Biology Reference
In-Depth Information
the wld-type they suggested that cyanopterin is involved in the inhibition of negative phototaxis
of the wild-type by sensing UV-A.
Fiedler
et al
. (2004) compared the growth performance of Cph1 and Cph2 mutants of
Synechocystis
sp. strain PCC 6803 under different light qualities and quantities. Cph1 mutant
showed a reduced growth under far-red light whereas the growth of Cph2 mutant strains was
inhibited by red light. On the other hand, Cph1 and Cph2 double mutants exhibited impaired
growth under high light conditions. Cph2 possesses the conserved Cys residues at positions Cys
129 and Cys 1022 that may be involved in sensing red/far red and blue light, respectively. Fiedler
et al
. (2005) demonstrated that the Cph2 mutants in which Cys1022 has been replaced with Val1022
showed photomovement of the cells towards blue light. Furthermore, the importance of Cys129 in
the N-terminal chromophore-binding domain of Cph2 has been revealed through the studies on
mutants lacking Cys129. Cph2 could perform its function independently and the presence of other
photoreceptos such as TaxD1 protein (similar in sequence to bacteriophytochrome and MCPs that
regulated phototxis; Yoshihara
et al
., 2000; Bhaya
et al
., 2001a) as well as a BLUF protein (product of
slr 1694 gene) of
Synechocystis
sp. strain PCC 6803 did not interfere with Cph2 function.
The mechanism of phototaxis in
Synechocystis
sp. strain PCC 6803 is explained on the basis of
existence of certain regulatory elements homologous to those described in bacterial chemotaxis
(Bhaya
et al
., 2001a). Three genetic loci specifi c to phototaxis (
tax1
,
tax2
and
tax3
) were identifi ed on
the basis of the isolation and characterization of 300 transposon-tagged mutants of
Synechocystis
sp.
strain PCC 6803. Of these 90% of them were immotile and the rest exhibited negative phototaxis.
The products of
tax1
and
tax3
are homologous to bacterial chemotaxis proteins and are related to
Tfp-mediated phototaxis (Bhaya
et al
., 2001b). They concluded that a single transposon insertion
in each mutant could be traced at different positions in the same genetic locus. Secondly, it was
easy to map the position of the inserted transposon because of the known genome sequence of
Synechocystis
sp. strain PCC 6803. This was confi rmed by the similar phenotype of these mutants
with mutants isolated after gene inactivation experiments (Bhaya
et al
., 1999, 2000, 2001b). TaxD1 has
been identifi ed as the photoreceptor in phototaxis of
Synechocystis
sp. strain PCC 6803 (Yoshihara
et al
., 2000; Bhaya
et al
., 2001a). TaxD1 has a sequence similarity to both bacteriophytochromes
and MCPs. It perceives light signals and triggers a chain of phosphorylation reactions involving
histidine kinase TaxAY1, which is classifi ed as a hybrid sensor histidine kinase has a fused CheY-like
regulator domain at its C-terminal end. This in turn modulates the activity of motility motor (Bhaya
et al
., 2001b). The
taxD3
locus (
taxD3
/
ctr1
/
pilJ
) has been identifi ed to encode a chemo-receptor-like
protein and a CheA-like histidine kinase (encoded by
taxAY3
). Both
taxAY3
and
taxD3
disruptant
mutants though yielded non-motile phenotypes with complete loss of Tfp, the presence of thin pili
on the cell surface still could be demonstrated by electron microscopy (Bhaya
et al
., 2001b; Chung
et al
., 2001; Yoshihara
et al
., 2002). TaxD3 lacks a sensing domain but possesses a tetratricopeptide
(TPR) domain at the N-terminus. Several of the mutants were affected in the structure of specialized
proteins with coiled-coil domains, or with TPR domains or certain others with tandem pentapeptide
repeat domains (Bhaya
et al
., 2001a). Interestingly,
taxD2
locus which contains all the components of
a chemotaxis-like system does not appear to be involved in motility (Yoshihara
et al
., 2001). Bhaya
(2004) summarized the current status on phototaxis in cyanobacteria and highlighted the necessary
gaps to be fi lled in this area. The wild-type cells of
Synechocystis
sp. strain PCC 6803 showed red
light-dominated action spectra (645 and 704 nm) for positive phototaxis whereas a mutant of the
same organism required blue light for negative phototaxis (Ng
et al
., 2003). It is concluded that while
positive phototaxis is controlled by TaxD1 protein, negative phototaxis is mediated by certain other
blue-light photoreceptors. It is important to note here that the 645 nm action spectrum reported for
