Master of Science
Daiki Kojima
Graduate School of Life Sciences, Bio-resilience research project (BRRP), Toyo University, Asaka, Saitama 351-8510 Japan
Title: Riboswitch Disruption Activates Gdx-Dependent Cesium Efflux in a Highly Cs⁺-Tolerant Eschelichia coli Strain ZX-1
Daiki Kojima1,2 and Masahiro Ito1,2,3
1 Graduate School of Life Sciences, Toyo University, Asaka, Saitama 351-8510 Japan
2 Bio-resilience research project (BRRP), Toyo University, Asaka, Saitama 351-8510 Japan
3 Bio Nano Electronics Research Center, Toyo University, Kawagoe, Saitama 350-8585 Japan
The Fukushima Daiichi nuclear accident in 2011 released large quantities of radioactive cesium (Cs) into the environment, making Cs⁺ removal a crucial target for bioremediation research. In Escherichia coli, Cs⁺ enters the cell primarily through the low-selectivity K⁺ transporter Kup, and no dedicated Cs⁺ efflux system has been reported to date. Excess intracellular Cs⁺ disrupts ionic homeostasis and severely inhibits cell growth(1).
In this study, we investigated the molecular basis of the exceptional Cs⁺ tolerance of the E. coli strain ZX-1, which was isolated in our laboratory(2). RNA-seq comparison between ZX-1 and its parental strain Mach1 revealed several uniquely upregulated genes in ZX-1. Among them, the guanidinium exporter Gdx was selected as a candidate Cs⁺ exporter, despite its known specificity for monovalent organic cations. The strong upregulation of gdx in ZX-1 was confirmed independently by RT-qPCR.
Heterologous overexpression of gdx in Mach1 using the pBAD24 vector conferred Cs⁺ tolerance exceeding that of ZX-1, and intracellular ion measurements demonstrated Cs⁺ reduction comparable to ZX-1. Functional Cs⁺/H⁺ antiport assays further established that Gdx actively exports Cs⁺, with an apparent Km of ~46.1 mM at pH 8.5.
Genome analysis of ZX-1 identified a 20 bp deletion upstream of the gdx ORF, located within a riboswitch known to regulate Gdx translation. This disruption likely leads to constitutive Gdx production, providing a mechanistic explanation for the enhanced Cs⁺ resistance of ZX-1.
Together, these findings uncover a previously unrecognized Cs⁺ efflux route in E. coli mediated by Gdx and demonstrate that riboswitch inactivation can generate high Cs⁺ tolerance. Moreover, the unexpected ability of Gdx to export an inorganic monovalent ion highlights new possibilities for understanding substrate recognition in multidrug and small-molecule transporters. (1) Bossemeyer, D. et. al., (1989). J Bacteriol. 171: 2219-2221. (2) Kojima, D. et. al., (2024). Front Microbiol. 14:1340033.
Biography of the presenting author
Daiki KOJIMA is a second-year Ph.D. candidate in the Graduate School of Life Sciences at Toyo University, Japan. He completed his master’s degree in just one year due to outstanding academic performance. His research achievements have been recognized through multiple awards at domestic and international conferences, including the Excellent Poster Presentation Award at the 2023 Meeting of the Kanto Branch of the Japanese Society for Bioscience, Biotechnology, and Agrochemistry (JSBBA); the Young Scientist Poster Award at the International Conference on Extremophiles 2024; the Excellent Oral Presentation Award at the 2025 JSBBA Kanto Branch Meeting; the Excellent Poster Award at the East Asia Extremophiles Conference 2025; and the Excellent Poster Award at the Annual Meeting of the Japanese Society for Extremophiles 2025.
In recognition of his academic accomplishments and research potential, he has been appointed as a JSPS Research Fellow (DC2) starting in 2025. His research focuses on Cs⁺ transport and Cs⁺ tolerance mechanisms in bacteria.
Presenting author details
Full name: Daiki KOJIMA
Contact number:
Email: s49102400033@toyo.jp
Category: (Oral presentation)