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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
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