HETEROCYST - Cyanobacteria - page 235

level of individual cells as conceived earlier in the heterocyst pattern formation. In situ localization of
nif2 was monitored in a strain (JE35) with nif2 :: lacZ fusion. After allowing normal heterocyst pattern
formation under aerobic conditions for 48 h, strain JE35 was incubated under anoxic conditions. The
existence of fl uorescence in the cells adjacent to heterocysts expressing nif2 makes it clear that the
pattern of hetrocyst formation has no relation to the expression of nif2 . The cross-functionality of
nitrogenase components of NifH1 and VnfH of A . variabilis ATCC 29413 was established by Pratte
et al . (2006) who isolated single ( nifH1 or vnfH ) and double mutants ( nifH1 and vnfH ) of A . variabilis
ATCC 29413. The nifH1 mutant could grow diazotrophically in the absence of molybdate because
of the expression and substitution by vnfH . Likewise, in the vnfH mutant the vanadium nitrogenase
could be functional because of the expression and substitution by nifH1 , it means the absence of
V-Fe co-factor is fulfi lled by the presence of Mo-Fe co-factor. On the other hand, the double mutant
of nifH1 and vnfH could not grow diazotrophically due to the absence of both Mo-Fe and V-Fe
co-factors with or without molybdate or vanadate (Pratte et al ., 2006). Transcriptional pattern of nifH1
and vnfH in A . variabilis ATCC 29413 revealed that the previously identifi ed tsps of both nifH1 and
vnfH1 did not drive the expression of the lacZ gene as the reporter gene. A further search upstream
the promoter regions resulted in the recognition of a promoter within nifV1 and a promoter upstream
of nifB1 contributed to the expression of nifHDK1 where the contribution of nifB1 promoter to the
transcript pool was considerably larger (Ungerer et al ., 2010).
Molecular cloning and sequence analysis of the gene ( fdxH ) encoding heterocyst ferredoxin
of Anabaena sp. strain PCC 7120 have been performed. Two major transcripts of fdxH (0.59 and
1.85 kb) have been detected at late stages of heterocyst differentiation with a tsp located at 132
bp upstream of fdxH (Bӧhme and Haselkorn, 1988). By site-directed mutagenesis of the positively
charged amino acid residues present on the surface of FdxH, Schmitz et al . (1993) identifi ed two
lysine residues located at positions 10 and 11 of FdxH of Anabaena sp. strain PCC 7120 that interact
with dinitrogenase reductase ( nifH ). Exchange of the two lysine residues of FdxH by Glu10 and
Ala11 of PetF in a chimeric protein PetF:FdxH with N-terminal half (58-amino acid residues) of the
former and C-terminal half (40-amino acid residues) of the latter resulted in a decreased affi nity
to nitrogenase. Three dimensional structure of [2Fe-2S] ferredoxin of Anabaena sp. strain PCC 7120
is very similar to the plant-type ferredoxins with the iron sulphur cluster positioned at the outer
edge of the molecule. The secondary structure revealed the presence of seven strands of β-pleated
sheet, two α-helices and seven type 1 turns (Jacobson et al ., 1993). Razquin et al . (1995) compared the
catalytic potential of FdxH, PetE and fl avodoxin from Anabaena sp. strain PCC 7120 in transferring
electrons to nitrogenase. Reconstitution experiments with nitrogenase showed the effectiveness
of only FdxH in transferring electrons to nitrogenase. FdxH and PetE could effectively transfer
electrons to nitrogenase in reconstituted experiments from heterocyst preparations under anaerobic
conditions. Flavodoxin proved to be important under iron-limited conditions and could effectively
mediate electron transfer from PSI to NADP + . petH , located away from the nif gene cluster on the
genome map of Anabaena sp. strain PCC 7120, encodes FNR in the heterocysts after a nitrogen step-
down. The FNR protein increased by 10-fold in the heterocysts when compared to vegetative cells
as revealed by immunoquantifi cation experiments. However, the purifi ed FNR from heterocysts
did not differ from the FNR from vegetative cells (Razquin et al ., 1996). Schrautemeier et al . (1994)
characterized the [2Fe-2S]-type of ferredoxin encoded by fdxH gene as well as a new 2[4Fe-4S]-type
of ferredoxin encoded by another fdxH gene, located immediately downstream of the former, in the
non-heterocystous cyanobacterium, Plectonema boryanum PCC 73110. The latter type of ferredoxin
has been reported for the fi rst time in cyanobacteria. When P . boryanum PCC 73110 was cultured
in presence of combined nitrogen, it expressed a [2Fe-2S]-type of ferredoxin (type-1) encoded by