Conserved Two-component Hik2–Rre1 Signaling Is Activated Under Temperature Upshift and Plastoquinone-reducing Conditions in the Cyanobacterium Synechococcus elongatus PCC 7942

Abstract

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.

Thiol redox switches regulate the oligomeric state of cyanobacterial Rre1, RpaA and RpaB response regulators

“…Rre1 has recently been shown to be phosphorylated in vivo under reduced PQ pool condition during thermal stress, possibly via its cognate sensor kinase Hik2. Phosphorylation of Rre1 leads to heat shock protein A ( hspA ) gene transcript accumulation under the same condition [11]. Interestingly, Bairagi et al.…”

“…Rre1 has recently been shown to be phosphorylated in vivo under reduced PQ pool condition during thermal stress, possibly via its cognate sensor kinase Hik2. Phosphorylation of Rre1 leads to heat shock protein A (hspA) gene transcript accumulation under the same condition [11]. Interestingly, Bairagi et al [11] also find some hspA transcript accumulation even in the absence of Rre1 phosphorylation, suggesting a phosphorylation-independent regulation of the hspA gene.…”

“…Phosphorylation of Rre1 leads to heat shock protein A (hspA) gene transcript accumulation under the same condition [11]. Interestingly, Bairagi et al [11] also find some hspA transcript accumulation even in the absence of Rre1 phosphorylation, suggesting a phosphorylation-independent regulation of the hspA gene. The oxidation-induced oligomerisation that we report here might represent such a phosphorylationindependent pathway for regulation of Rre1 activity.…”

“…Analysis of Synechocystis sp. PCC 6803 rre1 mutant revealed that it is required for transcriptional control of salt and heat tolerance genes [10,11]. Hik2 and its plant orthologue, Chloroplast Sensor Kinase (CSK), were recently shown to sense the redox state of the plastoquinone (PQ) pool via an iron-sulfur cluster [12].…”

Structure of the SigF1-dependent <i>pilA1</i> gene promoter and characterization of the light-activated response in the cyanobacterium <i>Synechococcus elongatus</i> PCC 7942

“…Whole genome sequence analysis of Synechococcus revealed that our laboratory wild-type strain (hereafter TUA, originally derived from stock in a laboratory at Tokyo University of Agriculture) harbors a C to A mutation in the sigF1 gene (synpcc7942_1510), generating a nonsense codon at the 149 th amino acid (Q149*; Figs. S2 and S3) (Bairagi et al, 2022;Watanabe et al, 2012).…”