Biology Reference
In-Depth Information
that are devoted to encode eukaryotic serine/threonine protein kinases whose function remains
unclear. Homologs of
kaiABC
in a cluster with
cikA
exist elsewhere, although there is no evidence
of a circadian system.
In
Synechocystis
sp. strain PCC 6803 the phosphorus sensing mechanism involves SphS (an
orthologue of PhoR of
E
.
coli
) and the response regulator SphR (an orthologue of PhoB of
E
.
coli
),
respectively. All phosphorus assimilation genes are regulated by these two gene products. As
in
E
.
coli
, SphS senses inorganic phosphorus (P
i
) levels in the environment and if it is low it gets
autophosphorylated and activates SphR. Phosphorylation of SphR in turn leads to activation of
genes connected with phosphorus assimilation. All such genes whose transcription is regulated by
SphR are collectively called as Pho regulons. The phosphorylated SphR activates transcription of two
operons (
sphX
-
pstS1
-
C1
-
A1
-
B1
-
B1'
and
pst2
-
C2
-
A2
-
B2
) that encode ABC-type high affi nity P
i
-uptake
systems (Hirani
et al
., 2001; Suzuki
et al
., 2004).
Synechococcus
sp. strain WH8102 has only fi ve sensor
histidine kinases and nine response regulator genes of which one (SYNW1598) is a pseudogene
(Palenik
et al
., 2003). In
S
.
elongatus
PCC 6301 there are 13 and 21 potential genes for sensory histidine
kinases and response regulators, respectively. There are another three genes of hybrid type. At least
half of these relate to phosphate sensing
sphS
(
syc0424c
) and
sphR
(
syc0533d
) genes that encode the
sensor kinase. A comparison of the genomes of
Synechococcus
sp. strain CC9311 and
Synechococcus
sp. strain WH8102 revealed 28 genes in the former (11 genes for histidine kinases and 17 genes for
response regulators) and a minimum of 14 genes in the latter (fi ve histidine kinase genes and 9
response regulator genes). The arguments put forward are that
Synechococcus
sp. strain CC9311 is
a coastal strain and the environment being highly variable it requires greater adaptation whereas
Synechococcus
sp. strain WH8102 is an open ocean strain and grows almost in constant environmental
conditions. That is why it possesses minimal regulatory functions.
Synechococcus
sp. strain CC9311
also apparently lacks a phosphate sensor-response regulator (seen in several cyanobacteria and
bacteria) because of availability of phosphate at high concentrations in the coastal environment.
In
T
.
elongatus
BP-1 there are 17 and 27 potential genes for histidine kinases and response
regulators, respectively. These relate to highly conserved genes for a drug sensor (
tlr0437
), a
phosphonate sensor (
tll0925
), a KaiC-interacting protein (
tlr0029
), a phytochrome-like circadian
input kinase (
tll0899
) and three motility related CheA-like proteins (
tlr0349
,
tll0568
and
tll1021
). A
total of 11 genes for Ser/Thr protein kinases have been identifi ed in
T
.
elongatus
BP-1. Genes for
protein phosphatases in this organism include GlnB phosphatase (
tlr2243
) and two genes for adenyl
cyclase (
tll2280,
tll2410
) (Nakamura
et al
., 2002). Nakamura
et al
. (2003) have assigned 76 genes for a
two-component system in
G
.
violaceus
PCC 7421 that includes 27 genes for sensor histidine kinases,
37 response regulators and 12 for hybrids of sensor histidine kinases and response regulators. Of
the 37 response regulators, 27 of them contained sequences for transcription factors. Except for the
protein kinase domains of the serine/threonine protein kinases (15 of them) noted in this organism,
these do not bear any resemblance to those known in other cyanobacteria such as
Synechocystis
sp. strain PCC 6803,
Anabaena
sp. strain PCC 7120 and
T
.
elongatus
BP-1. A total of 195 genes that
encode two-component signal transduction systems have been identifi ed in
Anabaena
sp. strain PCC
7120. Of these 71 genes each one has been allocated for sensory kinases and response regulators
and 53 genes for hybrid sensory kinase and response regulator. Of these, 85 of them are arranged
in clusters of 2 to 5 genes and 110 of them are present individually. The gene arrangement in a fi ve
gene cluster (
alr3155
-
alr3159
) is in the order of a sensory kinase-a response regulator-a phytochrome-
like sensory kinase, a response regulator-a sensor/regulator hybrid. Thirteen of the sensory kinases
bear a conserved domain that resembles serine/threonine kinase. It is interesting to note that genes
for two-component system (two genes for sensory kinases paired with response regulators and
