The ω subunit of RNA polymerase is essential for thermal acclimation of the Cyanobacterium Synechocystis sp. PCC 6803

The rpoZ gene encodes the small ω subunit of RNA polymerase (RNAP). A ∆rpoZ strain of the cyanobacterium Synechocystis sp. PCC 6803 grew well in standard conditions (constant illumination at 40 µmol photons m-2s-1; 32 °C; ambient CO2) but was heat sensitive and died at 40 °C. In the control strain , 71 genes were at least two-fold up-regulated and 91 genes down-regulated after a 24-h treatment at 40 °C, while in ∆rpoZ 394 genes responded to heat. Only 62 of these heat-responsive genes were similarly regulated in both strains, and 80 % of heat-responsive genes were unique for ΔrpoZ. The RNAP core and the primary σ factor SigA were down-regulated in control strain at 40 °C, but not in ΔrpoZ. In accordance with reduced RNAP content, the total RNA content of mild-heat-stress-treated cells was lower in control strain than in ΔrpoZ. Light-saturated photosynthetic activity decreased more in ΔrpoZ than in control strain upon mild heat stress. The amounts of Photosystem II and Rubisco decreased at 40 °C in both strains while PSI and the phycobilisome antenna protein allophycocyanin remained at the same level as in standard conditions. The phycobilisome rod proteins, phycocyanins, diminished during the heat treatment in ΔrpoZ but not in control strain, and the nblA1 and nblA2 genes (encode NblA proteins required for phycobilisome degradation) were up-regulated only in ΔrpoZ. Our results show that the ω subunit of RNAP is essential in heat stress because it is required for heat acclimation of diverse cellular processes. Cells from cyanobacteria Synechocystis sp. PCC 6803 named as control strain (CS) and RNA polymerase omega subunit inactivation strain, ΔrpoZ, were harvested (A730=1, 40 mL) after treated at 40 °C under continuous illumination, PPFD 40 µmol m-2s-1, for 24 h.rpoZ基因编码RNA聚合酶（RNA polymerase, RNAP）的小型ω亚基。蓝细菌集胞藻（Synechocystis sp. PCC 6803）的ΔrpoZ菌株在标准条件（恒定光照，光量子通量密度40 µmol·m⁻²·s⁻¹；32 ℃；环境CO₂浓度）下生长良好，但表现出热敏感表型，于40 ℃时死亡。对照菌株在40 ℃处理24小时后，共有71个基因至少上调2倍，91个基因下调；而ΔrpoZ菌株中共有394个基因响应热胁迫。其中仅62个热响应基因在两种菌株中的调控模式一致，且80%的热响应基因仅在ΔrpoZ菌株中特异性存在。对照菌株在40 ℃时，RNA聚合酶核心酶及初级σ因子SigA的表达量出现下调，但ΔrpoZ菌株未发生该变化。与RNA聚合酶含量降低的结果一致，经轻度热胁迫处理的对照菌株总RNA含量低于ΔrpoZ菌株。轻度热胁迫下，ΔrpoZ菌株的光饱和光合活性降幅较对照菌株更为显著。两种菌株在40 ℃时，光系统II（Photosystem II, PSII）及核酮糖二磷酸羧化酶/加氧酶（Rubisco）的含量均出现下降，而光系统I（Photosystem I, PSI）与藻胆体天线蛋白别藻蓝蛋白（allophycocyanin）的含量则维持与标准条件下一致的水平。热胁迫处理期间，ΔrpoZ菌株的藻胆体杆蛋白藻蓝蛋白（phycocyanins）出现减少，但对照菌株未出现该变化；且nblA1与nblA2基因（编码藻胆体降解所需的NblA蛋白）仅在ΔrpoZ菌株中出现上调。本研究结果表明，RNA聚合酶的ω亚基在热胁迫过程中至关重要，因其参与调控多种细胞进程的热适应过程。本研究中，分别命名为对照菌株（CS）与RNA聚合酶ω亚基失活菌株ΔrpoZ的集胞藻细胞，于持续光照（光合有效辐射光量子通量密度PPFD 40 µmol·m⁻²·s⁻¹）下经40 ℃处理24小时后，收集其菌体（A₇₃₀=1，体积40 mL）。