HETEROCYST - Cyanobacteria - page 225

Figure 6: Diagram of the network considered. Cells are organized in a cyclical manner. Direct interactions are represented by
solid lines and indirect interactions by dashed lines. Arrow heads indicate activation and vertical lines indicate inhibition.
Numbers indicate the strength of the interactions considered among the elements of the network. With the kind permission
of Z. P. Gerdtzen, Centre for Biochemical Engineering and Biotechnology, Department of Chemical Engineering and
Biotechnology, University of Chile, Av. Beauchef 850, Santiago 837-0448, Chile [Gerdtzen et al. (2009) BMC Bioinformatics 10
(Suppl 6): S16 doi:10.1186/1471-2105-10-S6-S16].
Mo-Fe co-factor whereas the nitrogenase from vnf gene cluster contains a vanadium-Fe co-factor.
The arrangement of genes in nif1 gene cluster is almost the same as in Anabaena sp. strain PCC 7120,
except that it does not contain the 55-kb element in fdxN gene and is expressed only in the heterocysts
with Mo-dependent nitrogenase produced under aerobic conditions (Thiel et al ., 1995). In the nif2
gene cluster that lacks the fdxN gene, the homologues of nifE1 and nifN1 are fused into a single ORF
nifEN2 . The nif2 encoded Mo-nitrogenase is operative in vegetative cells and heterocysts under strictly
anaerobic conditions (Fig. 9; Thiel et al ., 1995, 1997). The vnf cluster is expressed in the heterocysts
(Thiel, 1993, 1996). Although there are evidences for greater genome erosion of the endosymbiont
Nostoc azollae 0708, all the genes of the nif1 gene cluster are intact and are fl anked on either side by
the presence of transposase genes (Ran et al ., 2010). There is a greater degree of similarity in the
arrangement of genes of nif1 gene cluster between N . azollae 0708 and Anabaena sp. strain PCC 7120 on
the one hand and N . punctiforme ATCC 29133 (PCC 73102) on the other that also has nif1 gene cluster
(Fig. 8). In the toxic Cylindrospermopsis raciborskii CS-505 nif gene cluster comprises of a 15 kb region
with contiguous nifHDK operon and 13 other genes related to nitrogen fi xation. This cluster is similar
to the nif1 gene cluster of Anabaena sp. strain PCC 7120 and A . variabilis ATCC 29413 described above
but the gene organization is comparable to the nif2 gene cluster of A . variabilis ATCC 29413 and
N . azollae 0708 (Fig. 8). In C. raciborskii CS-505 a vnf gene cluster is also present at a different locus
(Stucken et al ., 2010). Raphidiopsis brookii D9, an isolate from Brazil, does not fi x nitrogen and so did
not reveal a nif gene cluster in its genome but the presence of coding sequences for coenzymes and
amino acid transport are present (Figs. 9 and 10; Stucken et al ., 2010).