DnaK2 Mediates a Negative Feedback Regulation of the Heat Shock Responsive Hik2-Rre1 Two-Component System in the Cyanobacterium Synechococcus Elongatus PCC 7942.

The two-component system (TCS) is a conserved signal transduction module in bacteria. The Hik2-Rre1 system is responsible for transcriptional activation upon high-temperature shift as well as plastoquinone-related redox stress in the cyanobacterium Synechococcus elongatus PCC 7942. As heat-induced de novo protein synthesis was previously shown to be required to quench the heat-activated response, we investigated the underlying mechanism in this study. We found that the heat-inducible transcription activation was alleviated by the overexpression of dnaK2, which is an essential homolog of the highly conserved HSP70 chaperone and whose expression is induced under the control of the Hik2-Rre1 TCS. Phosphorylation of Rre1 correlated with transcription of the regulatory target hspA. The redox stress response was found to be similarly repressed by dnaK2 overexpression. Considered together with the previous information, we propose a negative feedback mechanism of the Hik2-Rre1-dependent stress response that maintains the cellular homeostasis mediated by DnaK2.

A high-temperature sensitivity of Synechococcus elongatus PCC 7942 due to a tRNA-Leu mutation.

Certain mutations of the model cyanobacterium Synechococcus elongatus PCC 7942 during laboratory storage have resulted in some divergent phenotypes. One laboratory-stored strain (H1) shows a temperature-sensitive (ts) growth phenotype at 40 °C. Here, we investigated the reason for this temperature sensitivity. Whole genome sequencing of H1 identified a single nucleotide mutation in synpcc7942_R0040 encoding tRNA-Leu(CAA). The mutation decreases the length of the tRNA-Leu t-arm from 5 to 4 base pairs, and this explains the ts phenotype. Secondary mutations suppressing the ts phenotype were identified in synpcc7942_1640, which putatively encodes a NYN domain-containing protein (nynA). The NYN domain is thought to be involved in tRNA/rRNA degradation. Thus, the structural stability of tRNA-Leu is critical for growth at 40 °C in Synechococcus elongatus PCC 7942.

The circadian rhythm regulator RpaA modulates photosynthetic electron transport and alters the preferable temperature range for growth in a cyanobacterium.

Cyanobacterial strains can grow within a specific temperature range that approximately corresponds to their natural habitat. However, how the preferable temperature range for growth (PTRG) is determined at the molecular level remains unclear. In this study, we detected a PTRG upshift in a mutant strain of Synechococcus elongatus PCC 7942 lacking the circadian rhythm regulator RpaA. Subsequent analyses revealed that RpaA decreases the electron transport from photosystem I to NADPH. The change in electron transport likely inhibits H2 O2 generation under high-temperature conditions and contributes to the observed PTRG upshift in rpaA-deficient cells. The importance of the effects of the circadian rhythm regulator on the PTRG is discussed.

Identification and analysis of a principal sigma factor interacting protein SinA, essential for growth at high temperatures in a cyanobacterium Synechococcus elongatus PCC 7942.

Proteins that bind to RNA polymerase (RNAP) sigma factors play important roles in various transcriptional regulations. In this study, we identified a candidate of the principal sigma factor interacting protein in cyanobacteria, named SinA, based on a previous comprehensive protein interaction study (Sato et al., 2007) and analyzed this in the cyanobacterium Synechococcus elongatus PCC 7942. SinA is highly conserved among cyanobacteria and a knock out mutant showed defective growth at a usually permissive high temperature (40°C). Because this observation suggested SinA involvement in heat-inducible transcriptional activation, we examined heat-inducible protein gene hspA expression after temperature upshifts. The second-step induction disappeared after 15 min in the sinA mutant. In vivo pull-down experiments demonstrated the interaction between SinA and the principal sigma factor RpoD1. This SinA-RpoD1 complex was associated with an RNAP core enzyme under growth temperatures, but was dissociated after a temperature upshift. Based on these results, we propose a function of SinA to facilitate the substitution of the principal sigma factor with alternative sigma factors under heat-stressed conditions.