HETEROCYST - Cyanobacteria - page 177

at the two poles for the formation of polar nodules makes the differentiation complete. However, it
must be stated that the functional nature of heterocysts would only be refl ected by the expression
of nitrogen fi xation genes. All these processes are supported by signal transduction proteins and to
some extent sigma factor proteins. So as to keep the internal atmosphere of the heterocyst to nearly
microoxic conditions a simultaneous down-regulation of PSII and an up-regulation in respiratory
activity takes place. The following account summarizes the initial events of phycobiliprotein
degradation and the gene expression profi les during differentiation of heterocyst.
Soon after nitrogen step-down, there is a transient degradation of phycobiliproteins in all the
vegetative cells and after the differentiation of heterocysts the level of these pigments is regained
to the normal levels in A . cylindrica . The development of proheterocysts occurred within a period of
4.5 h and mature heterocysts appeared by about 14 h that coincided with the development of
nitrogenase activity around 13.5 h (Bradley and Carr, 1976). In case of unicellular cyanobacteria
phycobiliprotein degradation in response to nitrogen deprivation leads to chlorosis (Boersch, 1910;
Allen and Smith, 1969). The process of chlorosis has been very intensively investigated in these
forms and the complementation of a non-bleaching mutant led to the identifi cation of a gene nblA
of S. elongatus PCC 7942. NblA, the protein encoded by nblA , is responsible for bringing about the
degradation of phycobiliproteins in vivo (Collier and Grossman, 1994). While S. elongatus PCC 7942
has one nblA gene, the genome of Synechocystis sp. strain PCC 6803 has two homologues of nblA ,
nblA1 and nblA2 whose corresponding proteins are required for phycobiliprotein degradation in
vivo (Baier et al ., 2001). NblA is a small protein (6 kDa) of 65 amino acids arranged into two α-helices
assembled at an angle of 37º in an antiparallel V-shaped arrangement. Strauss et al . (2002) cloned
the nblA gene from Anabaena sp. strain PCC 7120 and the recombinant NblA produced in Escherichia
coli though existed as a dimer but formed trimers in solution. The crystal structure of NblA from
the same organism revealed it to be a dimer consisting of four-helices formed by two dimers (Fig.
2). Site-directed mutagenesis helped in the identifi cation of Leu51 and Lys53 at the C-terminal
region responsible for binding to the α-chains of phycocyanin (at Gln16-Leu39) and phycoerythrin
(at Leu19-Ser45) (Bienert et al ., 2005). The importance of other amino acid residues in NblA such as
Ser9, Arg22, Arg56 and Lys33 has been recognized for interaction with phycobilisomes (Dines et al .,
2008). Anabaena sp. strain PCC 7120 has two nblA genes, one on the chromosome designated as nblA
( asr4517 ) and the other on delta plasmid known as nblA - p ( asr8504 ). A . variabilis ATCC 29413 has only
one nblA gene. The amino acid sequence of NblA proteins of both these Anabaena strains is 100%
identical but the gene sequences have 96% identity (Baier et al ., 2004). Baier et al . (2004) reported the
essentiality of NblA for phycobiliprotein degradation in vivo but not for the development of fully
functional heterocysts. The nblA gene disruptant mutants for nblA and nblA-p of Anabaena sp. strain
PCC 7120 accumulated high levels of phycobiliproteins not only in the vegetative cells but also in
the fully developed heterocysts. Localization of the phycobiliproteins by the immunogold particle
technique revealed more phycocyanin in the heterocysts. The presence of nblAI gene just upstream
of cpeBA (that encodes phycoerythrin apoprotein), regulation of its expression and interactions
of NblAI with phycobilisomes has been reported (Luque et al ., 2003). The localization of NblA in
the differentiated heterocysts of Tolypothrix sp. PCC 7601 pointed out that NblA is a necessary co-
factor for degradation of phycobilisomes but not the triggering factor (Ochoa de Alda et al ., 2004).
In this connection, the interaction of NblA with ClpC, a heat shock protein (HSP100) belonging to
cyanobacterial Clp protease, assumes signifi cance (Karradt et al ., 2008).
The basic studies on protein turnover in N . muscorum cells subjected to nitrogen step-down
revealed that several sets of proteins are synthesized in vegetative cells but some are preferentially
allowed to be continued in proheterocysts. There existed a qualitative difference in the sets of proteins