HETEROCYST - Cyanobacteria - page 240

a difference in the allocation of C and N in between the older and newly formed heterocysts as the
latter had higher enrichments. The neck region of heterocysts, cell division septae and cyanophycin
granules represented sites most active assimilation and biosynthesis (Popa et al ., 2007).
v) Transport : The important proteins that are essential for maintaining the integrity of fi laments
have largely been identifi ed by the work on fragmentation mutants. This led to the the researches
on the probable modes of transport of substances in between vegetative cells and heterocysts. First
we describe the fragmentation mutants, the proteins identifi ed followed by means of transport
between the cells of a fi lament of Anabaena sp. strain PCC 7120. Though mutants of Anabaena sp.
strain PCC 7120 that have a tendency to fragment into few cells or even single cells under nitrogen-
defi cient conditions have earlier been isolated, the genes regulating them or complementing them
have not been identifi ed (Buikema and Haselkorn, 1991a; Khudyakov and Pinevich, 1992). Bauer et
al . (1995) characterized three such mutants, strain 129, strain 415 and a third strain from among the
lot isolated by their group (Buikema and Haselkorn, 1991a). Besides fragmenting into a single-celled
state, strain 129 shows signs of proheterocyst differentiation before fragmenting but is not capable
of performing aerobic nitrogen fi xation due to the presence of very few mature heterocysts. The
gene, identifi ed by complementation of wild-type DNA, fraC encodes a phenylalanine-rich protein
of 179 amino acid residues and this protein seems to be essential for the integrity of cell junctions.
A comparison of the fraC gene sequences of strain 129 and the wild-type showed two base changes,
one at position 1108 (from T to G in the threonine codon that is a silent mutation) and the other
at position 770 (a frame-shift caused due to deletion of a single base). Strain 415 also fragments
extensively after nitrogen step-down, differentiates proheterocysts and few heterocysts but is unable
to perform aerobic nitogen fi xation. Since fraC could not complement the mutation in strain 415, it
appeared to have arisen due to a mutation in a gene other than fraC . The identifi cation of another
gene fraH has been possible by the isolation of mRNA and corresponding cDNA synthesis from a
third mutant at 30 h after nitrogen step-down. Insertional inactivation of fraH gene sequence in the
wild-type resulted in a mutant phenotype resembling the original phenotype in showing slightly
longer fragments with proheterocyst and heterocyst formation in higher frequency than the other
two mutants described above. The wild-type showed the mRNA transcripts of fraH after 12 h of
transfer to a nitrogen-free medium. Nayar et al . (2007) identifi ed a gene upstream of hetR of Anabaena
sp. strain PCC 7120 and designated it as fraG ( alr2338 ) on the basis of the phenotype of the mutants
that undergo fragmentation and are defi cient in the synthesis of Hgls. The gene alr2338 has been
disrupted at positions +124 bp, +158 bp, +176 bp and +1770 bp relative to the translational start site.
In addition, a 592 bp portion of the fraG gene was replaced in the wild-type by the introduction of
Sp-Sm resistance cassette through a suicide plasmid (pAN120). The resultant mutant (UHM127) and
the fraG disruptant mutants exhibited the same phenotype, i.e. fragmentation of the fi laments to the
extent of being in a single celled stage within 24 h after nitrogen step-down and in their inability
to synthesize Hgls. The wild-type synthesizes heterocyst glycolipids after 15 h of nitrogen step-down
but by this time UHM127 did not synthesize the Hgls and did not undergo fragmentation. A gene
construct carrying promoter of fraG fused to gfp when introduced into wild-type through a shuttle
vector, the expression of GFP was found to be higher in the developing and mature heterocysts. The
gene product, FraG has been predicted to be an integral membrane protein (with 751 amino acid
residues) essential for maintaining fi lament integrity and heterocyst maturation.
A few investigations have been devoted to understand what constitutes the continuity
between vegetative cells and heterocysts or vice-versa, the periplasm of the two cells or the
microplasmodesmata present in between cell junctions. Contradicting evidences have been presented
with reference to periplasm as the route for transport of substances. Mariscal et al. (2007) favour