HETEROCYST - Cyanobacteria - page 239

by the uniform levels of the transcripts of glsF and the activity of Fd-GOGAT in all the vegetative
cells grown in all nitrogen sources but its absence in the heterocysts has been supported by the
results from enzyme activity, Western and Northern blot analyses. The presence of NADP-isocitrate
dehydrogenase activity in the heterocysts is suggested to be responsible for the generation of 2-OG
and NADPH and the latter is utilized to reduce ferredoxin. In the model presented by them they
envisaged that the 2-OG is transported to the adjacent vegetative cells and in exchange the heterocysts
receive a glutamate molecule. Furthermore, they suggested that the transport of fi xed nitrogenous
substance from heterocysts could as well be glutamine or arginine because the heterocysts have full
potential to synthesize arginine which could be incorporated into cyanophycin. The breakdown of
cyanophycin could serve as a rich source of arginine as the fi xed product that could be transported
into vegetative cells depending on the need (Martín-Figueroa et al ., 2000).
Now we shall discuss the transport of metabolites from heterocysts to vegetative cells and
vice-versa. Although it is mostly agreed that glutamine is the end product of ammonia assimilation,
whether the transported nitrogenous substance is glutamine as such or a derivative of it is not yet
clear. Another likely alternative molecule projected is Ala. Two investigations support this alternative.
Nitrogen starved cells of A . cylindrica (F) when exposed to 14 CO 2 the label most prominently
accumulated in glutamine when incubated under ammonium and Ala was the most labelled product
under N 2 (Lawrie et al ., 1976). The second investigation is that of Jüttner (1983) who similarly found
good amount of labelled Ala from the extracts of heterocysts when whole fi laments of A . cylindrica
(F) were exposed to 14 CO 2 for very brief periods (20 s). Ala can be formed through a transamination
reaction from glutamate (Wolk et al ., 1976; Meeks et al ., 1978). Furthermore, the existence of fi ve types
of amino acid transport systems viz., high affi nity system for basic amino acids, neutral amino acid
transport systems (N-I and N-II), low affi nity, passive transport system for basic amino acids and
low affi nity system for the uptake of acidic amino acids is known in cyanobacteria. The repression
of N-I transport system that is required for optimum diazotrophic growth in the heterocysts, the
expression of BgtA, a high affi nity basic amino acid uptake transporter and N-II in vegetative cells
as well as heterocysts, the specifi c release of Ala by mutants impaired in natB and natCA operon and
the requirement of functional heterocysts for maximum release of alanine all point out to the likely
transport of Ala from vegetative cells to heterocysts (Montesinos et al ., 1995, 1997; Picossi et al ., 2005;
Pernil et al ., 2008). Moreover, the catabolic function of a compartmentalized alanine dehydrogenase
has been demonstrated by the expression of ald ( alr2355 ) gene fused with gfp as a reporter gene
inside the heterocysts of Anabaena sp. strain PCC 7120. Thus Ala catabolism appears to be essential
for diazotrophic growth (see Amino acid transport and diazotrophic growth).
With regard to carbon metabolism occurring in the heterocysts, the operation of a sucrose cycle
of synthesis (mediated by the expression of spsB and sppA ) and catabolism (mediated by invB ) and
its linking to glycogen turnover during nitrogen-fi xing conditions has come to light (Cumino et al. ,
2007). Further, the regulation of genes of sucrose biosynthesis by NtcA amply signifi es that besides
exercising nitrogen control, it also regulates carbon metabolism (Marcozzi et al. , 2009). All these have
been dealt in detail under the section on Carbon metabolism. The above aspects discussed make it
clear that heterocysts have the potential to build up the required carbon and nitrogen intermediates
required for diazotrophic growth, at the same time depending on the requirement carbon as
well as nitrogen inputs are received from vegetative cells. But exactly what is the nature of these
intermediates awaits further research. An attempt made in this direction by Popa et al . (2007) merits
mention. Carbon and nitrogen fi xation and the probable metabolite exchange between heterocysts
and vegetative cells of A . oscillarioides after enrichments with 13 CO - 3 and 15 N 2 revealed that newly
fi xed nitrogen is rapidly exported from heterocysts and located in the vegetative cells. There was