Biology Reference
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and Golden (2001) identifi ed genes
sigD
,
sigE
and
sigF
in
Anabaena
sp. strain PCC 7120. Insertional
inactivation of the three sigma factor genes has been achieved by the transfer of suicide plasmids
pAM2178 (carrying Ω Sp-Sm resistance cassette) or pAM2179 (with Cm
r
Em
r
cassette) fused with
lacZ
as the reporter gene through conjugation with
E
.
coli
that permitted blue-white colony screening on
X-gal plates. Inactivation of any of these genes did not signifi cantly affect the growth of the mutants
in nitrate nitrogen but mutants of
sigD
and
sigE
did not perform well under diazotrophic growth
conditions. A
sigDsigE
double mutant differentiated proheterocysts after nitrogen step-down but
had undergone extensive fragmentation. Likewise, another
sigEsigF
double mutant also established
poorly under nitrogen limitation and showed extensive bleaching. However, it is signifi cant to note
that the expression of the three sigma factor genes was quite feeble after a nitrogen step-down and
heterocyst differentiation in
Anabaena
sp. strain PCC 7120 as evidenced by the luminescence from
luxAB
reporter (Khudyakov and Golden, 2001). A group 2 sigma factor gene,
sigC
(
sll0184
) has been
shown to control the expression of
glnB
(that encodes PII protein) in the stationary phase nitrogen-
limited cultures of
Synechocystis
sp. strain PCC 6803 (Asayama
et al
., 2004). Aldea
et al
. (2007) identifi ed
the probable role of eight sigma factor genes, four each belonging to group 2 (
sigB2
,
sigC
,
sigD
and
sigE
), and group 3 (
sigF
,
sigG
and
sigI, sigJ
) during heterocyst differentiation. The up-regulation of
sigC
,
sigE
(previously
sigF
) and
sigG
during heterocyst differentiation of
Anabaena
sp. strain PCC
7120 at 4 h, 16 h and 9 h, respectively has been demonstrated by using transcriptional fusions with
gfp
as the reporter gene. Five other sigma factor genes
sigB2
,
sigD
,
sigI
and
sigJ
were expressed in
vegetative cells of nitrate-grown fi laments but after nitrogen step-down their expression was found
in both vegetative cells as well as heterocysts. Transcriptional fusions of
gfp
with the promoters of
sigC
,
sigE
and
sigG
revealed important information on the expression of genes in the development
of heterocyst. The expression of
sigC
coincided with the expression of important heterocyst-specifi c
early genes
hetC
,
hetP
,
devH
,
patS
,
patA
and
patB
;
hep
genes (
hepA
,
hepB
,
hepC
, and
hepK
) and Hgl genes
(
hglC
,
hglD
,
hglE
,
hglK
,
hglB
,
hetN
and
hetI
). The genes governing the synthesis of envelope layers
and respiratory terminal oxidases are expressed around 9 h corresponding with the expression of
sigG
. The expression of
sigE
around 16 to 17 h of nitrogen step-down suggests its involvement in
the transcription of
nif
genes that are expressed at late stages of heterocyst formation.
D) Signal transduction genes
Two-component, one-component and Ser/Thr and Tyr protein kinases constitute three major signal
transduction systems. Of these three types, the former two have been well characterized in the
prokaryotes while the third Ser/Thr or Tyr kinases, widely distributed in eukaryotes, are the newly
identifi ed systems in prokaryotes. In the fi rst type, there are two components; one is a sensor kinase
that transfers phosphate from a His residue to an Asp residue in the response regulator (Stock
et
al
., 1990; Parkinson, 1993). One-component systems have known input and ouput domains but
lack histidine kinase and the receiver domains. These are the major signal transduction systems
identifi ed in prokaryotes (Ulrich
et al
., 2005). The current status on the purported role of Ser/Thr
protein kinases and phosphatases in changing growth conditions, cell metabolism, photosynthesis
and stress responses in cyanobacteria has been reviewed (Zhang, C-C.
et al.,
2005; Zhang, X.
et al.,
2007). Among the genomes so far sequenced, except for some unicellular forms (four strains of
Prochlorococcus
and one marine
Synechococcus
WH8102) Ser/Thr protein kinases are present in the rest
of them but abundantly represented in fi lamentous, nitrogen-fi xing forms. In
Anabaena
sp. strain PCC
7120 fi ve Ser/Thr protein kinases have been characterized. These are PknA (alr4366; Zhang, 1993),
