HETEROCYST - Cyanobacteria - page 247

subunits for a periplasmic-binding and transmembrane domains encoded by alr3187 . N-II constitutes
the second transporter for acidic and neutral polar amino acids. NatF, a periplasmic substrate-
binding protein and NatG and NatH are transmembrane proteins of N-II transporter. The ATPase
subunit BgtA serves both the transporters. Bgt and N-II are expressed both in the vegetative cells
and heterocysts. Since Bgt is dispensable for diazotrophic growth, N-I that is exclusively expressed
in vegetative cells (Picossi et al. , 2005) together with N-II constitute the major transport systems for
98% of the amino acids and are important for diazotrophic growth.
The main conclusions of the above investigations are that: (i) heterocysts export amino acids to
the periplasmic space from where they would diffuse out before being taken up by vegetative cells;
(ii) through N-I transporter the vegetative cells take up a number of amino acids and recapture a
number of hydrophobic amino acids leaked out from the cells; (iii) there is a differential expression
of the nat genes in the vegetative cells and heterocysts; (iv) since continuity of periplasmic space in
between vegetative cells and heterocysts of a fi lament is maintained, there would be an exchange of
these molecules between the two cell types; (iv) though Gln is projected as the product of nitrogen
fi xation that is transferred to vegetative cells, there exists a possibility for Ala being considered as
a candidate because synthesis of Ala in the heterocysts has been demonstrated (Lawrie, 1976; Wolk
et al ., 1976; Jüttner, 1983); (v) the release of Ala by natB mutants of Anabaena sp. strain PCC 7120
requires functional heterocysts and (vi) Ala can be a good substrate for alanine dehydrogenase and
a number of other tranaminases.
Alanine dehydrogenase (Ald; EC 1.4.1.1) is an NAD-dependent enzyme that catalyzes the
reversible deamination of L-Ala to pyruvate, ammonia, NADH and H + . Ald is a hexameric protein
and the molecular mass ranges from 270 kDa as in A . cylindrica (Rowell and stewart, 1975) to 240
kDa as in Phormidium lapideum (Sawa et al ., 1994). The molecular mass of the subunits accordingly
is 43 kDa in the former and 41 kDa in the latter. Ald from P . lapideum does not possess any sequence
similarity with other dehydrogenases but it is closely related to the family of 2-hydroxyacid
dehydrogenases. The subunits of Ald get folded into two identical domains that are separated
by a cleft. A region of (120) amino acids that is highly conserved is clustered around the cleft that
might contain the active site of the enzyme (Sedelnikova et al ., 1998; Baker et al ., 1998). Ald activity
is required for phycobilisome degradation in S . elongatus PCC 7942 when subjected to nitrogen
starvation. Inactivation of ald gene of S . elongatus 7942 by insertion of a Sp r cassette resulted in an
Ald-defi cient mutant that exhibited down-regulation of a number of nitrogen stress-related genes,
foremost of which are nblA and nblC . The wild-type cells showed high Ald activity upon nitrogen
starvation to maintain low cellular levels of L-Ala while the Ald-mutant accumulated higher levels
of L-Ala and in the absence of induction of nblA and nblC phycobilisome degradation did not
occur despite nitrogen defi ciency (Lahmi et al ., 2006). An L-amino acid oxidase has been identifi ed
associated with thylakoid membranes of Synechocystis sp. strain PCC 6803. It can oxidize a number
of basic L-amino acids with L-Cys and L-Arg serving as preferred substrates (with 150% and 100%
activity when L-Arg is taken as standard) from among a number of others, the oxidation of L-Ala
being 37%. This amino acid oxidase can also function as L-Arg dehydrogenase mediating electron
transfer from L-Arg to the respiratory electron transport (Schriek et al ., 2009). Such amino acid
oxidases with dual function are yet to be demonstrated in the heterocystous cyanobacteria. However,
Ala catabolism seems to be essential for the diazotrophic growth of Anabaena sp. strain PCC 7120
because in an ald (ORF alr2355) inactivated mutant the diazotrophic growth potential decreased by
50%. A strain with ald - gfp gene construct showed the expression of ald in the developing and mature
heterocysts (Pernil et al ., 2010).