Conserved Two-Component Hik2-Rre1 Signalling is Activated Under Temperature Upshift and Plastoquinone-Reducing Conditions in the Cyanobacterium Synechococcus elongatus PCC 7942.

The highly conserved Hik2-Rre1 two-component system is a multi-stress responsive signal-transducing module that controls the expression of hsp and other genes in cyanobacteria. Previously, we found in Synechococcus elongatus PCC 7942 that the heat-inducible phosphorylation of Rre1 was alleviated in a hik34 mutant, suggesting that Hik34 positively regulates signalling. In this study, we examined the growth of the hik34 deletion mutant in detail, and newly identified suppressor mutations located in rre1 or sasA gene negating the phenotype. Subsequent analyses indicated that heat-inducible Rre1 phosphorylation is dependent on Hik2 and that Hik34 modulates this Hik2-dependent response. In the following part of this study, we focused on the mechanism to control the Hik2 activity. Other recent studies reported that Hik2 activity is regulated by the redox status of plastoquinone (PQ) through the 3Fe-4S cluster attached to the GAF domain. Consistent with this, Rre1 phosphorylation occurred after addition of 2,5-dibromo-6-isopropyl-3-methyl-1,4-benzoquinone (DBMIB) but not after addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) to the culture medium, which corresponded to PQ-reducing or -oxidising conditions, respectively, suggesting that the Hik2-to-Rre1 phosphotransfer was activated under PQ-reducing conditions. However, there was no correlation between the measured PQ redox status and Rre1 phosphorylation during the temperature upshift. Therefore, changes in the PQ redox status are not the direct reason for the heat inducible Rre1 phosphorylation, while some redox regulation is likely involved as oxidation events dependent on 2,6-dichloro-1,4-benzoquinone (DCBQ) prevented heat-inducible Rre1 phosphorylation. On the basis of these results, we propose a model for the control of Hik2-dependent Rre1 phosphorylation.