Biology Reference
In-Depth Information
Figure 8:
Illustration of genes related to N
2
-fi xation, a highly conserved gene cluster in cyanobacteria. The structural genes
for the nitrogenase enzyme (
nifHDK
) are highlighted in color for clarity. Also, genes which differ in terms of occurrence
and/or organization are indicated in grey. The nitrogenase enzyme catalyzes the fi xation of atmospheric dinitrogen gas.
Transposases are indicated in red. Three dots indicate gaps and incision elements, with the length of the omitted sequence
given. With the kind permission of B. Bergman, Department of Botany, Stockholm University, Stockholm, Sweden [Ran
et al
.
(2010)
PLoS ONE
5(7):
e11486. doi:10.1371/journal.pone.0011486] doi:10.1371/journal.pone.0011486.g007.
Color image of this figure appears in the color plate section at the end of the topic.
organism to complete the differentiatiation of heterocysts and fi x nitrogen under aerobic conditions.
Golden
et al
. (1985) fi rst reported the excision of the 11-kb element fl anking the
nifK
and
nifD
genes
precisely around 18 h after nitrogen step-down during heterocyst differentiation in
Anabaena
sp.
strain PCC 7120. This results in juxtaposition of the two genes and continuity of the
nifD
protein-
coding sequence coming under the transcriptional control of
nifH
promoter region (Haselkorn
et al
.,
1983, 1986). In many of the non-heterocystous diazotrophic cyanobacteria that have been examined
there is no 11-kb element in the
nifD
gene and the
nifKDH
is a contiguous region. Examples include
Plectonema
(Barnum and Gendel, 1985),
Cyanothece
sp. and
Synechococcus
sp. (Kallas
et al
., 1985) and
Pseudoanabaena
ATCC 29210 and
Oscillatoria tenuis
UTEX 1566
(Saville
et al
., 1987) that fi x nitrogen
under microaerophilic or anaerobic conditions. Non-nitrogen fi xing unicellular cyanobacteria like
Cocchochloris peniocystis
ATCC 27147,
Aphanocapsa
sp. ATCC 27178 and
Synechocystis
sp. ATCC 29109
neither contain the
nifKDH
nor the 11-kb sequences in their genomes (Saville
et al
., 1987). The genomes
of many heterocystous cyanobacteria such as strains of
Anabaena
,
Nostoc
and
Calothrix
contain the
11 kb element. However, exceptions do exist that do not contain the 11 kb element in their genomes
such as
Fischerella
sp. ATCC 27929 (Saville
et al.
, 1987) and certain strains of
Anabaena
symbiotic in
A
.
caroliniana
and
Anthoceros punctatus
(Franche and Cohen-Bazire, 1985, 1987; Nierzwicki-Bauer
and Haselkorn, 1986; Meeks
et al.
, 1988),
N
.
azollae
0708 endosymbiont of
A
.
fi liculoides
(Ran
et al
.,
2010) and
C
.
raciborskii
CS-505 (Stucken
et al
., 2010). However, differences in the freshly isolated
and cultured strains of the symbionts of
Azolla
and
A
.
punctatus
do exist. For example, in the