Biology Reference
In-Depth Information
The discovery of a number of regulatory genes (
ntcA
,
hetR
,
hetA
,
hetP
,
hetC
,
hetF
,
hetL
,
hetY
,
hetZ
and
asr1734
), development genes (
devBCA
operon,
devH
,
devR
and
devT
), those of Hgls and Hep
layer and pattern formation genes (
patA
,
patB
,
hetN
and
patS
) has been made by the characterization
of different classes of mutants that are defi cient in heterocyst development and nitrogen fi xation.
Moreover, the participation of genes for sigma factors and signal transduction in expression of the
above genes or clusters of genes has also been indicated. A redeeming feature is that most of the
knowledge in these areas has been generated by the use of mostly a single cyanobacterium,
Anabaena
sp. strain PCC 7120. A number of molecular tools and processes have been developed to pursue the
gene expression profi les. The following account highlights the developments in these areas.
8) MUTANTS DEFECTIVE IN HETEROCYST DEVELOPMENT AND NITROGEN
FIXATION
N-methyl-N'-nitro-N-nitrosoguanidine (NTG-)-induced mutants of
N
.
muscorum
defi cient in
heterocyst differentiation
and nitrogen fi xation have been put to use in genetic recombination
experiments (Stewart and Singh, 1975). NTG-induced mutants of
N
.
muscorum
and
N
.
linckia
have been
characterized with respect to het
-
, nif
-
(heterocyst-less, non-nitrogen fi xing), het
+
, nif
-
(heterocystous
and non-nitrogen-fi xing) and M-het
+
, nif
+
(multiple heterocystous) phenotypes. The involvement of
a common genetic determinant(s) for heterocyst differentiation and the expression of
nif
genes in
both the organisms and for heterocyst spacing pattern in
N
.
linckia
have been suggested (Singh
et
al
., 1977). A large number of mutants (65) of
A
.
variabilis
ATCC 29413 that required inorganic fi xed
nitrogen sources for growth have been isolated after NTG-mutagenesis. These showed altered pattern
of heterocyst differentiation (with low and high frequency of heterocysts), with thick or incompletely
developed heterocyst envelopes and protoplasts separated from heterocysts (Currier
et al
., 1977). Some
of these mutants of
A
.
variabilis
ATCC 29413 defi cient in Hgls showed very low or no nitrogenase
activity when grown under aerobic conditions suggesting a direct correlation between the absence of
Hgls in the laminated layer and O
2
-induced damage. The revertants of this class of mutants showed
normal synthesis of Hgls and aerobic nitrogen fi xation. Other classes of mutants in which Hgls were
normal either required a supply of reductant in the form of dithionite or microaerophilic conditions
for nitrogen fi xation (Haury and Wolk, 1978). A short-trichome mutant of
N
.
muscorum
exhibiting
het
-
, nif
-
phenotype with reduced growth rate than the wild-type even in nitrate-medium has been
reported that reverted to wild-type with a frequency of 2.62 x 10
-7
(Padhy and Singh, 1978). Species
of
Nostoc
(
N
.
linckia
),
Anabaena
(
Anabaena
strain CA) and
Gloeotrichia
(
G
.
ghosei
) have been put to
use for the isolation of such mutants that showed altered pattern of nitrogen fi xation and required
the exogenous supply of fi xed nitrogen sources for their survival (Tiwari, 1977; Tiwari
et al
., 1979;
Gotto
et a
l., 1979; Grillo
et al
., 1979). Some of the NTG-induced mutants of
G
.
ghosei
showed the
phenotypic features with loss of both polarity and heterocyst differentiation, longer fi laments with
normal heterocysts and chains of heterocysts (Mishra and Tiwari, 1986). Heterocystless mutants
of
Anabaena
sp. strain PCC 7120 exhibited whole cell acetylene reduction activity under aerobic or
microaerophilic conditions but exhibited similar growth pattern in the fi xed nitrogen sources as well
as no alteration in either glutamine synthetase [GS; L-glutamate:ammonia ligase (ADP-forming); EC
6.3.1.2] and glutamate synthase [GOGAT; L-glutamate:oxoglutarate amidotransferase; EC 1.4.7.1]
activities (Spence and Stewart, 1987). Although a physiological and biochemical analysis of the
above mutants is available, information on genes that regulate either the formation of heterocysts
or causes for defi ciency in nitrogen fi xation is not available.
