Transcript levels of rbcR1, ntcA, and rbcL/S genes in cyanobacterium Anabaena sp. PCC 7120 are downregulated in response to cold and osmotic stress

Using differential display, we identified the Anabaena sp. PCC 7120 ribulose 1,5-bisphosphate carboxylase transcriptional regulator (rbcR1) gene, a member of the LysR family of positive transcription factors. The rbcR1 transcript and its putative target gene ribulose 1,5-bisphosphate carboxylase/oxygenase (rbcL/S) were repressed by cold (20°C) and osmotic (sucrose and salt) stress. Cold stress also induced a transient downregulation of the Anabaena 7120 ntcA transcriptional regulator. Expression of the ntcA gene, however, returned to normal levels 2 h after initiation of cold stress and increased significantly above normal levels 24 h after growth at 20°C. The early decline in the expression of the ntcA, rbcR1, and rbcL/S transcripts appears to be part of the Anabaena 7120 global adaptation response to stress. The substantial increase in the ntcA gene expression 24 h following cold stress suggests that Anabaena 7120 experiences substantial nitrogen limitation under these conditions. These data suggest that in response to stress, Anabaena 7120 decreases its metabolic activity through regulation of the CO₂ fixation machinery while enhancing its nitrogen assimilation by inducing the expression of the nitrogen global transcriptional regulator, NtcA.