Biology Reference
In-Depth Information
6126 of 6129 ORFs of
Anabaena
sp. strain PCC 7120 genome and hybridization was conducted. In
terms of cluster analysis and functional categories of genes 329 ORFs were differentially expressed.
Seven clusters of ORFs have been recognized. In cluster 1, most of the genes were down-regulated
(mentioned in perentheses) in
hepK
(167),
hepN
(117),
hepS
(65) and
henR
(53) mutants and most of
the ORFs in
hepK
and
hepN
mutants pertained to '
nif
' and '
pol
' (polysaccharide envelope) islands. In
the cluster 2, 18 genes were all down-regulated in the
hepN
mutant and most of these (12) pertained
to 'Hep' island and 10 of these were down-regulated in
hepS
mutant as well. In cluster 3, in the
henR
and
hepS
mtants 13 and 10 genes respectively mostly belonging to 'Hgl' island were down-regulated.
In cluster 4, 9 genes were down-regulated in the
hepK
mutant. Cluster 5 contains 70 ORFs that were
up-regulated mostly in
hepK
and/or
hepN
mutant. Cluster 6 consists of seven ORFs that were all
up-regulated in the
hepK
mutant, of these up-regulation of
furA
is noteworthy. In cluster 7, there were
23 ORFs up-regulated in
hepN
mutant. HepS and HenR appear to belong to a common regulatory
pathway because of the regulation of common set of genes under their control. The expression of
patA
gene in all the four mutants except in
hepK
mutant signifi es that
patA
expression is inhibited
by products related to heterocyst development.
C) Genes for sigma factors
Sigma factors of RNA polymerase help in transcription of genes by binding to appropriate recognition
sequences in the promoter region and facilitate open complex formation. There are two families
of sigma factors, i.e. σ
70
and σ
54
. The σ
70
family has been divided into three groups. Group 1 sigma
factors or primary sigma factors that control transcription of housekeeping genes and are essential
for survival. Group 2 sigma factors are very much similar to group 1 but are non-essential. Group
3 sigma factors are also known as alternative sigma factors and they are specifi c for transcribing
mRNAs for production of proteins in specialized organs (of motility) or processes such as stress
responses, endospore formation and extra-cytoplasmic functions. Sigma factors of the family σ
54
are
not represented in cyanobacteria.
Anabaena
sp. strain PCC 7120 possesses 12 putative genes for sigma
factors, nine of them are located on the chromosome (group 1:
sigA
,
all5263
; group 2:
sigB2
,
alr3800
;
sigC
,
all1692
;
sigD
,
alr3810
; and
sigE
,
alr4249
; group 3:
sigF
,
all3853
;
sigG
,
alr3280
;
sigI
,
all2193
and
sigJ
,
alr0277
) and three of them are on the plasmids (
sigB
,
all7615
;
sigB3
,
all7608
;
sigB4
,
all7179
). Yoshimura
et al
. (2007) presented a phylogenetic analysis of sigma factors of four (
Anabaena
sp. strain PCC
7120,
A
.
variabilis
ATCC 29413,
Synechocystis
sp. 6803 and
S. elongatus
PCC 7942) cyanobacteria. The
importance of group 2 sigma factors during a change in nutrients, light and temperature conditions
in cyanobacteria has been highlighted (Osanai
et al
., 2008). A primary sigma factor encoding gene
sigA
has been isolated and characterized by Brahamsha and Haselkorn (1991). Two transcripts of
1.7 kb and 2.2 kb are encoded by the monocistronic
sigA
gene with the former remaining constant
under nitrogen-enriched as well as nitrogen-depleted conditions while the latter showed enhaced
levels of its production after nitrogen step-down in
Anabaena
sp. strain PCC 7120. Studies on cloning,
expression and inactivation of
sigB
and
sigC
genes of
Anabaena
sp. strain PCC 7120 revealed that
SigB (with 332 amino acid residues) and SigC (with 416 amino acid residues) are produced only
under nitrogen-limited conditions but these are not required either for nitrogen fi xation or heterocyst
differentiation (Brahamsha and Haselkorn, 1992). During a search of DNA-binding proteins that
are able to bind to the promoter region of
rbcL
gene, Ramasubramanian
et al
. (1994a) isolated
sigA
gene of
Anabaena
sp. strain PCC 7120. A mutation in
sigH
of
N
.
punctiforme
, a symbiont of
Anthoceros
punctatus
, enhanced the infection potential by 6-fold with a simultaneous increase in nitrogenase
activity without any apparent increase in the heterocyst frequency (Campbell
et al
., 1998). Khudyakov
