Biology Reference
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constitutively at low levels. The expression of three other genes tested (
hetM
,
hglE
and
hetN
) was
unaffected. Targeted inactivation of the gene in the wild-type resulted in the same mutation and
complementation of the mutant with wild-type copy restored the phenotype. Fan
et al
. (2006)
identifi ed three additional signal transduction genes required for heterocyst maturation in
Anabaena
sp. strain PCC 7120 during characterization of 1076 Fox
-
mutants generated by transposon (Tn5-1063)
insertion mutagenesis. These are a transcriptional regulator,
conR
(
all0187
), a response regulator,
henR
(
alr1086
) and a Ser/Thr kinase,
hepS
(
all2760
). They also confi rmed the observations of Ning
and Xu (2004a) regarding the gene
alr0117
and in consultation with them named this gene as
hepN
.
The regulatory nature of HenR is exemplifi ed by the nature of the pleiotropic effects it exerted on
the envelope polysaccharide and heterocyst glycolipid layers. So it is suggested that HenR is a part
of master system that regulates synthesis of heterocyst envelope that may be bifurcated into one
controlling deposition of glycolipid layer and the other regulating the deposition of polysachharide
layer. (The properties of the mutant phenotypes of these four regulatory genes are described under
section late genes or maturation genes).
patB
gene of
Anabaena
sp. strain PCC 7120 encodes a predicted protein of 529 amino acid residues
and its N-terminal region bears similarity to Fe4-S4 bacterial-type ferredoxins while the C-terminal
region is in the form of helix-turn-helix, typical of transcriptional regulators. In PAT-2 mutant,
heterocysts appeared after a delay 42 h when subjected to nitrogen-step -down.
patB
sequences are
found in other species of
Anabaena
,
Nostoc
and
Calothrix
and absent in unicellular cyanobacteria
and other fi lamentous cyanobacteria that fi x nitrogen under anaerobic conditions (Liang
et al
.,
1993). There are as many as 118 genes that probably encode transcriptional regulators in
Anabaena
sp. strain PCC 7120 (Ohmori
et al
., 2001). Koksharova and Wolk (2002) reported four more genes
abp1
,
abp2
,
abp3
and
abp4
that encode DNA-binding proteins. The existence of
hetR
as homodimer,
its DNA-binding activity and its inhibition by PatS have been reported by Huang
et al
. (2004).
Ehira and Ohmori (2006a) employed 5336 oligonucleotide probes in a DNA microarray analysis
and found up-regulation in the transcript levels of 410 genes at one or more time points (3, 8 and
24 h) after nitrogen step-down of
Anabaena
sp. strain PCC 7120. Of these, transcripts of ten genes
from among the 108 presumptive transcriptional regulators were up-regulated prominently with
transcripts of
all4312
superceding the rest. The rest nine genes up-regulated have been found to be
transcriptional regulators (
alr2178
,
alr2208
,
alr2597
,
alr2625
and
all4925
), two-component response
regulator (
all3788
),
hetR
(
alr2329
),
devH
(
alr3592
; a transcriptional regulator) and
ntcA
(
alr4392)
. The
ORF
all4312
has been designated as
nrrA
(nitrogen responsive regulator) that encodes a response
regulator, of the OmpR family. These results have further been confi rmed by conducting real-time
reverse transcription PCR. The presence of specifi c NtcA-binding region in the promoter region of
nrrA
indicates that this gene is regulated by NtcA. The NrrA levels were enhanced within 3 h of
nitrogen step-down. Heterocyst differentiation was delayed in a
nrrA
deletion mutant up to 24 h of
nitrogen step-down due to lower levels of
hetR
induction (Ehira and Ohmori, 2006a). Simultaneouly,
Muro-Pastor
et al
. (2006) published similar results on the role of gene product of
all4312
. Their
investigation adduced additional evidences for the suggested role of All4312. Expression of
all4312
occurred in all cells including proheterocysts and heterocysts as evidenced by the fl uorescence of
GFP when
gfp
was used as the reporter gene with promoter region of
all4312
. In the
hetR
mutant
strain, the expression of
all4312
was not altered. NtcA binding to the NtcA-binding region located
in the promoter region of
all4312
at 41.5 nucleotides with respect to tsp is enhanced in presence of
2-oxoglutarate. Ehira and Ohmori (2006b) showed that NrrA directly regulates expression of
hetR
during heterocyst differentiation in
Anabaena
sp. strain PCC 7120 as demonstrated by its binding
in
vitro
to the nitrogen responsive tsps located at -728 and -696 positions. This suggests that transcription
