DNA hybridization using the Synechocystis PCC6803 glucose transporter gene, glcP, revealed a single homologous region in two facultative photoautotrophic strains out of three tested, and none in three obligate autotrophs. In one of the latter, Synechococcus PCC7942, integration of glcP into the chromosome resulted in glucose sensitivity. A subclone isolated as glucose-tolerant had lost glcP. Integration in a replicative vector allowed glucose transport and photoheterotrophic growth, but could not be maintained. Thus lack of sugar transport could explain cyanobacterial obligate autotrophy. However, at least in Synechococcus PCC7942, acquisition of such a transport capacity created a metabolic disequilibrium barely compatible with survival.

Reversible protein phosphorylation plays important roles in signal transduction. One gene, prpA, encoding a protein similar to eukaryotic types of phosphoprotein phosphatases PP1, PP2A, and PP2B, was cloned from the nitrogen-fixing cyanobacterium Anabaena sp. strain PCC 7120. Interestingly, a eukaryotic-type protein kinase gene, pknE, was found 301 bp downstream of prpA. This unusual genetic arrangement provides the opportunity for study about how the balance between protein phosphorylation and dephosphorylation can regulate cellular activities. Both proteins were overproduced in Escherichia coli and used to raise polyclonal antibodies. Immunodetection and RNA/DNA hybridization experiments suggest that these two genes are unlikely to be coexpressed, despite their close genetic linkage. PrpA is expressed constitutively under different nitrogen conditions, while PknE expression varies according to the nature of the nitrogen source. Inactivation analysis in vivo suggests that PrpA and PknE function to ensure a correct level of phosphorylation of the targets in order to regulate similar biological processes such as heterocyst structure formation and nitrogen fixation.

Anabaena sp. PCC 7120 is a cyanobacterium capable of performing several important biological functions: photosynthesis, nitrogen fixation, cell differentiation, cell-cell communication, etc. These activities require an extensive signaling capability in order to respond to the changing environment. Based on the genomic data, we have retrieved several gene families encoding signaling components. It is estimated that 211 genes encode two-component signaling elements, and 66 genes encode Ser/Thr kinases and phosphatases. These genes together represent 4.2% of the coding capacity of the whole genome, making Anabaena PCC 7120 a leading member among prokaryotes in terms of its signaling potential. It is known that two-component systems are composed of a few basic modules that can arrange into different structures best adapted for each signaling system. Many proteins in Anabaena PCC 7120 have incorporated both modules of two-component systems and catalytic domains of either Ser/Thr kinases or phosphatases. A family of 13 genes encode proteins with both a Ser/Thr kinase domain and a His kinase domain, and another four genes were also found whose products have both a response regulator domain and a Ser/Thr phosphatase domain. Of all the signaling proteins in Anabaena PCC 7120, about one third (35%) are conserved in the genome of the unicellular cyanobacterium strain Synechocystis sp. PCC 6803. Interestingly, one subfamily of His kinases and two subfamilies of response regulators are found in Anabaena PCC 7120 but are absent in Synechocystis PCC 6803. This study constitutes a basis for analyses of signal transduction in Anabaena PCC 7120 using functional genomic approaches.

In cyanobacteria, the isiA gene is required for cell adaptation to oxidative damage caused by the absence of iron. We show here that a putative Ser/Thr kinase gene, pkn22 (alr2052), is activated by iron deficiency and oxidative damage in Anabaena sp. PCC 7120. A pkn22 insertion mutant is unable to grow when iron is limiting. pkn22 regulates the expression of isiA (encoding CP43P), but not of isiB (encoding flavodoxin) and psbC (CP43). Fluorescence measurement at 77 K reveals the absence of the typical signature of CP43P associated with photosystem I in the mutant under iron-limiting conditions. We propose that Pkn22 is required for the function of isiA/CP43P and constitutes a regulatory element necessary for stress response.

Heterocysts are terminally differentiated cells devoted to nitrogen fixation in the filamentous cyanobacterium Anabaena sp. strain PCC 7120. We show here that the cell division protein FtsZ is present in vegetative cells but undetectable in heterocysts. These results provide a first rational explanation for the inability of mature heterocysts to undergo cell division.