[The gap3 gene of Synechococcus PCC 7942 is induced during adaptation to low CO2 concentrations]

The gap3 genes of the Synechococcus and Anabaena cyanobacteria fulfill so far unknown function. A homolog of this gene has recently been found in the nuclear genomes of diplonemids, which are heterotrophic flagellates closely related to kinetoplastids and euglenoids. To understand the function of the gap3 gene in the cyanobacteria, we performed Northern blotting experiments with the gap3 probes under different growth conditions. Under the standard photosynthetic growth conditions (high illumination and 1% CO2 in the gas phase), the expression of the gap3 gene was very low, but significantly increased during cell adaptation to the low CO2 concentration (0.03%). The gap3 operon was expressed as a polycistronic transcript of about 7 kb in size, which included ORF2 (1259 bp) immediately downstream of gap3. ORF2 probably encodes a putative transporter of HCO3. The nucleotide sequence of ORF2 has been submitted to GenBank under accession number AF 428100.