HETEROCYST - Cyanobacteria - page 200

The phenotypes of these two mutants resembled that of UCD311 in being Fox - except that in presence
of combined nitrogen extensive fragmentation of the fi laments resulted in short 2-3 celled fragments
(Hagen and Meeks, 1999).
In Anabaena sp. strain PCC 7120, alr4674 encodes DevT that resembles a Ser/Thr protein
phosphatase. Three important features of devT are that (i) it is not under the regulation of NtcA,
(ii) its expression is to some extent inhibited by HetR and (iii) it accumulates specifi cally in mature
heterocysts, though initially it is constitutively expressed in all cells. In view of these, Espinosa et
al . (2010) concluded that DevT plays an unknown role in heterocyst maturation.
ii) Genes for Heterocyst-specifi c Glycolipids : Amongst a number of mutants of Anabaena sp. strain
PCC 7120 isolated by Buikema and Haselkorn (1991b), the characterization of strain 543 led to
the identifi cation of a gene, hglK responsible for the transport of heterocyst glycolipids outside
the cells and their assembly into the laminated layer (Black et al ., 1995). Strain 543 differentiated
heterocysts without the laminated layer of glycolipids though the Hgls are synthesized by the
cells. The N-terminal portion of HglK protein is organized into four membrane spanning domains
which help the protein to anchor to the cell membrane and the C-terminal portion consists of 36
pentapeptide repeats. The fi rst and third amino acid residues of the pentapeptide repeats, Ala and
Leu, respectively are highly conserved while the rest three are variable, i.e. AXLXX. Cloning and
sequencing of the PCR-generated fragments of the strain 543 genomic DNA revealed that a base
change at the amino acid residue 496 resulted in a stop codon just before the pentapeptide repeat
due to which the C-terminal portion got deleted. Accordingly, a mutant, constructed by the insertion
of Sp-Sm resistance cassette just at the beginning of the hglK gene, also showed a similar phenotype
as that of strain 543. The hglK gene seems to be highly conserved and restricted to the heterocystous
cyanobacteria in its distribution (Black et al. , 1995). Bauer et al . (1997) described the presence of hglB
( alr5357 ), hglC ( alr5355 ) and hglD ( alr5354 ) upstream of hetN in Anabaena sp. strain PCC 7120. These
three genes are shown to be essential for Hgl synthesis as null mutations in any one of the three
genes resulted in strains that are capable of differentiating heterocysts devoid of a Hgl laminated
layer. Cohen et al . (1994) isolated a Tn5-1063 transposon mutant of N . punctiforme ATCC 29133
with luxAB as reporter gene (strain 307) that failed to develop mature heterocysts with glycolipid
laminated layer but fi xed nitrogen under anoxic conditions. Cloning and sequence analysis of the
mutant genome revealed it to be an ORF of 3,159 bp and this has been designated as hglE by Campbell
et al . (1997). The putative gene product of hglE is a protein of 115.4 kDa. HglE protein consists of
active sites characteristic of polyketide synthases (PKS) with a consensus motif GPX 5 TACSS and
also possesses 16 highly conserved amino acids within a region of 200 amino acids that constitutes
the PKS domain. The presence of hglE gene sequence exclusively in heterocystous cyanobacteria
( A . variabilis ATCC 29413, Anabaena sp. strain PCC 7120, Nostoc sp. strain ATCC 27896, Nostoc sp.
strain MAC, N . punctiforme ATCC 29133, Nostoc sp. strain ATCC 29106, and other free-living and
symbiotic Nostoc strains) and the inactivation of which resulted in the failure to form Hgl layer led
them to believe that the synthesis of Hgls is mediated through the PKS pathway (Campbell et al .,
1997). The upregulation of hglE ( alr5351 ) and another ORF alr5347 by sodium chloride in Anabaena
sp. strain PCC 7120 and its absence in the adenyl cyclase C (CyaC; this is an important enzyme which
mediates the synthesis of cAMP) disruptant mutant demonstrates that sodium chloride-induced
enhancement in gene expression leads to the deposition of Hgls (Imashimizu et al ., 2005).
Maldener et al. (2003) characterized fi ve mutants of Anabaena sp. strain PCC 7120, of which T22
and P2 are important to be noted in connection with Hgls. In case of mutant T22, the ORF alr2887
has been shown to be responsible for export of Hgls. This mutant formed heterocyst envelope layer