Possible involvement of cyanobacteria in the formation of plant hormonal system

It was shown for the first time the presence of the soluble cytokinin-binding protein with a mol wt of 67 kD (CBP67) in the cyanobacterium Synechocysti sp. PCC 6803. CBP67 was isolated by two independent methods: by affinity chromatography on zeatin-Sepharose and by immunoaffinity chromatography on zeatin-Sepharose using monoclonal antibodies (mAb) raised against maize CBP70 (70 kD). The cytokinin-binding properties of CBP67 were assessed by its interaction with anti-idiotype Ab (Abₐ₋ᵢ) raised against Ab to zeatin. Abₐ₋ᵢ are, in fact, Ab to zeatin-binding site of the protein. The high specificity of interaction between trans-zeatin and CBP67 was demonstrated. Zeatin riboside and cis-zeatin mahifested a reduced interaction with CBP67; other phytohormones (auxin, gibberellic acid, and ABA) and adenine did not interact with the protein at all. Cyanobacterial CBP67 activated in vitro transcription in the Synechocystis lysate. The highest effect on transcription was observed at a combined action of CBP67 and trans-zeatin. Cyanobacterial CBP67 activated transcription in the barley chloroplast lysate as well. The totality of the results indicates a possibility of the existence of systems of cytokinin signal perception in the evolutionary precursor of chloroplasts, cyanobacteria, which could bring this system into a plant cell.