Phosphorus deficiency alleviates cyanobacterial iron-limitation through a direct PhoB_x0002_ mediated gene regulation

Iron (Fe) and phosphorus (P) are essential nutrients with close geochemical association. They exist at low concentrations in surface waters and may be co-limiting resources for phytoplankton growth. However, the adaptive strategies of photosynthetic organisms to Fe/P co-limitation remain largely unknown. Here, we show that phosphorus deficiency increases the growth of Fe-limited cyanobacteria through a PhoB-mediated regulatory network. In addition to its well-recognized role in controlling phosphate homeostasis, PhoB regulates key metabolic processes crucial for Fe-limited cyanobacteria, including ROS detoxification and Fe uptake. Transcript abundances of PhoB-targeted genes are enriched in samples from the P-deplete ocean, and a conserved PhoB binding site is widely present in the promoters of the targets, suggesting that the strategy we discovered may be highly conserved. Our findings provide important molecular insights into the response of cyanobacteria to simultaneous Fe/P nutrient limitation and help in understanding how nutrient availability affects primary productivity in aquatic environments. Overall design: The phoB-FLAG3 strains were grown in phosphate limited medium to an OD730 of 0.5 and then cross-linked by 1% formaldehyde treatment for 15 min at room temperature,followed by quenching with 125 mM glycine. Sample preparation for ChIP-Seq was performed as previously described and libraries were prepared from two biological replicates. The library sequencing was performed with Illumina NovaSeq 6000 by IGENEBOOK Biotechnology Co., Ltd. The MACS2 program (version 2.1.1.20160309) was used for peak calling, and the data were visualized by IGV (version 2.16.0)

铁（Fe）与磷（P）是地球化学关联紧密的必需营养元素，二者在地表水中浓度较低，且可共同成为浮游植物生长的限制因子。然而，光合生物应对铁/磷共同限制的适应策略目前仍鲜为人知。
本研究表明，磷匮乏可通过PhoB介导的调控网络促进铁限制蓝细菌的生长。除了已被广泛认知的调控磷酸盐稳态的功能外，PhoB还可调控铁限制蓝细菌中的关键代谢过程，包括活性氧（Reactive Oxygen Species, ROS）解毒与铁摄取。PhoB靶基因的转录本丰度在贫磷海洋样本中显著富集，且靶基因启动子区域广泛存在保守的PhoB结合位点，提示本研究揭示的适应策略可能具有高度保守性。
本研究为蓝细菌响应铁、磷协同营养限制的分子机制提供了重要见解，有助于理解营养可利用性如何影响水生环境中的初级生产力。
整体实验设计：将phoB-FLAG3菌株接种于磷限制培养基中培养至730 nm光密度值（OD730）为0.5，随后于室温下用1%甲醛交联15分钟，再以125 mM甘氨酸终止交联反应。染色质免疫共沉淀测序（Chromatin Immunoprecipitation Sequencing, ChIP-Seq）的样本制备参照既往研究方法，文库构建采用两个生物学重复。文库测序由IGENEBOOK生物技术有限公司基于Illumina NovaSeq 6000平台完成。采用MACS2软件（版本2.1.1.20160309）进行峰调用，数据可视化采用IGV软件（版本2.16.0）