HETEROCYST - Cyanobacteria - page 178

Figure 2: Free NblA dimer (from the crystal structure) and how it could penetrate into the gap formed between two hexamers
of phycoerythrocyanin rod of Anabaena sp. strain PCC 7120. Picture courtesy of Noam Adir, Schulich Faculty of Chemistry ,
Technion-Israel Institute of Technology , Technion City, Haifa 32000 , Israel.
Color image of this figure appears in the color plate section at the end of the topic.
expressed in proheterocysts which was apparent after 9 h of nitrogen limitation (Fleming and
Haselkorn, 1974). In order to understand the number of genes that are expressed from proheterocyst
to mature heterocyst formation in A . variabilis ATCC 29413, Lynn et al . (1986) conducted DNA:RNA
hybridization studies. Three important aspects that emerged from these studies are: (i) the expression
of the genome was higher in vegetative cells (65%) than in the heterocysts (45%); (ii) the number of
heterocyst-specifi c mRNA transcripts was approximately equivalent to 1000 genes; (iii) a constitutive
expression of about 900 to 1300 genes occurred in both vegetative cells and heterocysts and (iv)
the frequency of abundance of transcripts in heterocysts was higher than those in the vegetative
cells. Lynn and Ownby (1987) confi rmed the high turnover of mRNA in the isolated heterocysts of
A . variabilis ATCC 29413 that continued for 2 h but decreased thereafter. However, in heterocysts
of intact fi laments the transcript accumulation continued as noted earlier (Lynn et al ., 1986). The
enhanced gene expression patterns of A . variabilis ATCC 29413 after 8.5 h of nitrogen starvation could
be correlated with the increase of nitrogenase activity that continued for another 5 to 6 h coinciding
with the appearance of mature heterocysts. Simultaneously, the levels of transcripts for phycocyanin
and allophycocyanin decreased within the fi rst hour of nitrogen starvation. This is because of the
short half-lives (16 to 18 min) of the transcipts of both cpc and apc genes. This decrease was soon
made up by a rapid breakdown of proteins followed by enhanced nitrogen fi xation (Wealand et
al ., 1989). Synthesis of dinitrogenase reductase and its even distribution in the newly differentiated
heterocysts of the symbionts of Azolla fi liculoides , A. caroliniana , and A . pinnata coincided with the
formation of contorted membranes and polar nodules (Braun-Howland et al ., 1988). But it was with