Spx is a central regulator in different organisms. In Bacillus subtilis, it's a redox-responsive transcription factor and a key regulator of the stress response. It binds to the alpha subunits of RNA polymerase, regulating the expression of stress response genes and interfering with developmental processes. It also represses ribosomal RNA transcription, linking stress response to translation control [1,3]. In plants, SPX-domain-containing proteins are crucial for maintaining phosphate (Pi) homeostasis, involved in processes like Pi transport and adaptation to Pi deficiency [2,4,5,6,7,8,9].

In Bacillus subtilis, Spx drives the expression of a large regulon in response to proteotoxic conditions such as heat, disulfide stress, and cell wall stress. In plants, genetic and biochemical studies of SPX-domain proteins have revealed their role in regulating the activity of central regulators like AtPHR1/OsPHR2 in a Pi-dependent manner at different subcellular levels. For instance, in Arabidopsis, a SPX-domain vacuolar transporter (VPT1) has an auto-regulatory mechanism for Pi sensing and homeostasis [3,2,9].

In conclusion, Spx has diverse essential functions. In bacteria, it's vital for stress response and the connection between stress and translation control. In plants, it plays a key role in phosphate homeostasis and signaling. These functions, revealed through various studies, help us understand the biological processes in these organisms, with potential implications for improving plant nutrient use efficiency and understanding bacterial stress-related mechanisms.

References:

1. Schäfer, Heinrich, Turgay, Kürşad. 2019. Spx, a versatile regulator of the Bacillus subtilis stress response. In Current genetics, 65, 871-876. doi:10.1007/s00294-019-00950-6. https://pubmed.ncbi.nlm.nih.gov/30830258/

2. Zhou, Zhipeng, Wang, Zhiye, Lv, Qundan, Wu, Ping, Mao, Chuanzao. . SPX proteins regulate Pi homeostasis and signaling in different subcellular level. In Plant signaling & behavior, 10, e1061163. doi:10.1080/15592324.2015.1061163. https://pubmed.ncbi.nlm.nih.gov/26224365/

3. Rojas-Tapias, Daniel F, Helmann, John D. 2019. Roles and regulation of Spx family transcription factors in Bacillus subtilis and related species. In Advances in microbial physiology, 75, 279-323. doi:10.1016/bs.ampbs.2019.05.003. https://pubmed.ncbi.nlm.nih.gov/31655740/

4. Liu, Na, Shang, Wenyan, Li, Chuang, Xing, Guozhen, Zheng, WenMing. . Evolution of the SPX gene family in plants and its role in the response mechanism to phosphorus stress. In Open biology, 8, . doi:10.1098/rsob.170231. https://pubmed.ncbi.nlm.nih.gov/29298909/

5. Jung, Ji-Yul, Ried, Martina K, Hothorn, Michael, Poirier, Yves. 2017. Control of plant phosphate homeostasis by inositol pyrophosphates and the SPX domain. In Current opinion in biotechnology, 49, 156-162. doi:10.1016/j.copbio.2017.08.012. https://pubmed.ncbi.nlm.nih.gov/28889038/

6. Shi, Jincai, Zhao, Boyu, Zheng, Shuang, Yu, Nan, Wang, Ertao. 2021. A phosphate starvation response-centered network regulates mycorrhizal symbiosis. In Cell, 184, 5527-5540.e18. doi:10.1016/j.cell.2021.09.030. https://pubmed.ncbi.nlm.nih.gov/34644527/

7. Zhang, Kaidian, Zhou, Zhi, Li, Jiashun, Yu, Liying, Lin, Senjie. 2021. SPX-related genes regulate phosphorus homeostasis in the marine phytoplankton, Phaeodactylum tricornutum. In Communications biology, 4, 797. doi:10.1038/s42003-021-02284-x. https://pubmed.ncbi.nlm.nih.gov/34172821/

8. Secco, David, Wang, Chuang, Arpat, Bulak A, Shou, Huixia, Whelan, James. . The emerging importance of the SPX domain-containing proteins in phosphate homeostasis. In The New phytologist, 193, 842-51. doi:. https://pubmed.ncbi.nlm.nih.gov/22403821/

9. Luan, Mingda, Zhao, Fugeng, Sun, Guangfang, Lan, Wenzhi, Luan, Sheng. 2022. A SPX domain vacuolar transporter links phosphate sensing to homeostasis in Arabidopsis. In Molecular plant, 15, 1590-1601. doi:10.1016/j.molp.2022.09.005. https://pubmed.ncbi.nlm.nih.gov/36097639/