HETEROCYST - Cyanobacteria - page 242

now been identifi ed by Merino-Puerto et al . (2010). Of the four Fra proteins identifi ed, i.e. FraC
( alr2392 ), FraD ( alr1603 ), FraE ( alr2394 ), FraH ( alr1603 ), the fi rst three of them are encoded by the
respective genes as a single operon constitutively and fraH is induced under nitrogen deprivation. The
localization of FraC and FraD at the intercellular septa has been confi rmed by the gfp transcriptional
fusions with the promoter regions of fraC and fraD . So the three proteins FraG, FraC and FraD help in
organization of the intercellular septa. The observations of Mariscal et al . (2011) assume signifi cance
in defi ning the role of FraG as an intercellular conduit for molecular exchange. The recognition of
three domain structure for FraG (or SepJ; an N-terminal coiled-coil domain, a central linker and a
C-terminal permease domain) and deletion mutants of the three domains focused on the importance
of coiled-coil domain in the localization of FraG protein at the intercellular septa and for maintenance
of the integrity of the fi laments. Expression of conR ( all0187 ), up-regulated in proheterocysts and
heterocysts at 9 h after nitrogen step- down of Anabaena sp. strain PCC 7120, with P conR - gfp as reporter
gene revealed that this gene plays an important role in septum formation. Mutants of ConR have been
found to be defective in the septum formation and differentiated longer heterocysts with their polar
nodules partially open. Though the mutant possessed nitogenase and fi xed nitrogen to the extent
of 70% of the wild-type, the defi ciency of diazotrophic growth has been due to lack transportation
of fi xed nitrogen to the vegetative cells (Mella-Herrera et al., 2010).
vi) Carbon metabolism : The activities of G6P-dehydrogenase and 6-phosphogluconate hydrogenase
were (6 to 8-fold) higher in isolated heterocysts of A . cylindrica (Winkenbach and Wolk, 1973; Lex and
Carr, 1974). Due to this reason heterocysts are suggested to receive a source of reductant in the form
of NADPH for nitrogenase to function in light as well as dark (Bothe, 1970). G6P-dehydrogenase is
encoded by zwf gene and its inactivation in Nostoc sp. strain ATCC 29133 caused failure of diazotrophic
growth in light or in dark in presence of organic carbon sources. The zwf transcript was expressed as
an operon and contained sequences for genes encoding fructose-1,6-biphosphatase, transaldolase as
well as an undesignated gene known as opcA . Complementation of the wild-type genes restored the
diazotrophic growth potential signifying that in light OPP pathway provides the requisite reductant
for nitrogen fi xation and respiration in the heterocysts (Summers et al ., 1995). Enzymes of glycolysis
and part of TCA cycle occur in heterocysts (Bothe, 1982). In addition, the presence of pyruvate-
ferredoxin oxidoreductase (Neur and Bothe, 1982), isocitrate dehydrogenase (Papen et al ., 1983) and
glutamate-oxoglutarate amidotransferase (Hӓger et al ., 1983) has been demonstrated. Due to the
presence of all the enzymes for the conversion of G6P to oxoglutarate, heterocysts are not dependent
on vegetative cells for a supply of glutamate (Neuer and Bothe, 1983; Papen et al ., 1986).
CO 2 -fi xation by the isolated heterocysts of A . cylindrica is negligible due to lack or low levels of
RUBP-carboxylase (Fay and Walsby, 1966; Winkenbach and Wolk, 1973; Lex and Carr, 1974; Codd
et al ., 1980) and as such in the absence of CO 2 -fi xation, the required reduced carbon compounds
are supplied by the adjacent vegetative cells. Wolk (1968) demonstrated that when A . cylindrica
fi laments were exposed to radioactive carbon, the movement of carbon label moved from vegetative
cells to the heterocysts. But the nature of carbon compounds that move from vegetative cells into
the heterocysts to support nitrogen fi xation remained sketchy for a long time. However, persistent
efforts were made in this direction. 14 CO 2 -labelling of the fi laments of A . cylindrica revealed that the
heterocysts isolated after short intervals of 20 s showed labelled alanine, glutamate and glutamine
and G6P. When alanine was supplied to the isolated heterocysts it got readily converted to glutamate
and glutamine under N 2 -H 2 atmosphere. It was thus concluded that alanine serves as a precursor for
reducing equivalents required by the heterocysts (Jüttner, 1983). A survey of 33 organic compounds
that included organic acids, polyhydric alcohols, sugars and sugar phosphates identifi ed D-erythrose