SYMBIOSIS - Cyanobacteria - page 381

leaves of A . fi liculoides by the use of immunoelectron microscopy and immunogold labelling. Most
of the label was found in chloroplasts but in hair cells abundant label was noted both in chloroplasts
and in the cytoplasm. The existence of weak label in the hairs of the cyanobiont-free plants points
towards the important role of hair cells in the assimilation of ammonia.
Peters and Calvert (1983) suggested that the regulation of GS in the association is at the level of
function rather than synthesis. According to Orr and Haselkorn (1982) the GS activity in the symbiotic
association was barely detectable either by applying enzyme assays or by radioimmuno assay.
These are 5-10% lower in the symbiont in situ than in the free-living form. Molecular mechanisms
governing gene activation and gene repression during heterocyst differentiation in cyanobacteria
indicate that regulation is affected at the level of transcription (see chapter 4). Differences in mRNA
levels of genes for nitrogenase and GS have been studied in the endosymbiont of Azolla by northern
hybridization by using the corresponding cloned genes from Anabaena sp. strain PCC 7120. The nif
genes ( nifH , nifD , and nifK ) are transcribed from a single nif HDK operon. A lowering of GS transcript
levels up to 10% was found in the endosymbiont compared with the levels in free-living A . azollae
(Nierzwicki-Bauer and Haselkorn, 1986). This leads to the conclusion that the host plant some how
regulates transcription of the Anabaena glnA gene. In bacteria glnA transcription is regulated by the
ratio of glutamine to ketoglutarate, a high ratio of these leads to repression. If the same mechanism
is operative in the Azolla - Anabaena symbiosis, in that case it is of no use for the fern to transport
glutamine to the symbiont as that will eventually be metabolized. Nierzwicki-Bauer and Haselkorn
(1986) concluded that the fern might produce a non-metabolizable analog of glutamine that may act
as a co-repressor of the Anabaena glnA gene but which does not affect the eukaryotic gene.
Photosynthetic effi ciency of the symbiont of Azolla has been studied. A . azollae accounted for
7.5 to 15% of the total chlorophyll content of the symbiosis (Peters and Mayne, 1974a). Symbionts
isolated from Azolla could fi x CO 2 (Peters, 1975). Photosynthetic characterization of the symbiotic
association and that of individual partners indicated that the fi xation of CO 2 occurs via the Calvin
cycle but the role of the symbiont contributing to the photosynthesis in the intact association was
not apparent in the action spectrum (Ray et al ., 1979). A comparison of photosynthetic rates of Azolla -
Anabaena symbiosis and Anabaena -free Azolla plants suggested that Anabaena contributes very little
to carbon fi xation. The symbiont is dependent on the host for its carbon that is supplied in the form
of sucrose. Due to a fi ve-fold decrease in the level of transcription of RuBisCO in the symbiont of
Azolla , Nierzwicki-Bauer and Haselkorn (1986) concluded that the regulation of transcription of
rbcL - rbcS operon may be at the level of repression by the host through a diffusible carbohydrate.
ATP necessary to perform nitrogen fi xation is synthesized photosynthetically in the heterocysts of
the symbiont (Kaplan and Peters, 1981). Further, the heterocysts of the symbionts also are shown
to possess RuBisCO, the activity of which is generally absent in the heterocysts but present in
vegetative cells of free-living cyanobacteria. Thus it has been possible to detect the presence of rbcL
and rbcS genes in heterocysts and vegetative cells of Anabaena spp. in association with or isolated
from Azolla - Anabaena symbiosis by the application of in situ hybridaztion studies. However, the
functional role of RuBisCO in heterocysts of the symbionts of Azolla remains to be elucidated (Madan
and Nierwicki-Bauer, 1993).
Studies on genome erosion of the symbiont N . azollae 0708 brought out a number of new
observations that explain the symbiosis in Azolla . Due to the accumulation of large number of
pseudogenes (31.2%) and the presence of more than 600 insertion sequences in many of the genes,
regulating important functional areas of DNA replication, repair, glycolysis and biosynthesis of co-
factors of the endosymbiont, have become redundant. Furthermore, other processes that are impaired
relate to the uptake of bicarbonate, phosphate, nitrate and urea. However, due to the presence of