A magnesium transporter is involved in the cesium ion resistance of the high-concentration cesium ion-resistant bacterium Microbacterium sp. TS-1

Abstract

Cesium ion (Cs+) resistance has been reported in bacteria but is poorly understood as reports on Cs+-resistant bacteria have been limited. We previously reported a novel Cs+/H+ antiporter CshA implicated in Cs+-resistance in Microbacterium sp. TS-1. The present study used the same screening method to isolate novel Cs+-sensitive mutants and their revertants from TS-1. A comparative mutation site analysis using whole-genome sequencing revealed that MTS1_03028 encodes the Mg2+ transporter MgtE and is a candidate Cs+ resistance-related gene. We performed a bioinformatic analysis of MTS1_03028 and complementation experiments on Cs+ resistance in the TS-1 MTS1_03028 mutants Mut5 and Mut7 as well as Escherichia coli expressing MTS1_03028 in the presence of Mg2+. We established the role of MgtE in Cs+ resistance through a functional analysis of TS-1. Enhancing Mg2+ transport by expression of MTS_03028 conferred increased Cs+ resistance. When this strain was exposed to Cs+ concentrations exceeding 200 mM, CshA consistently lowered the intracellular Cs+ concentration. To our knowledge, the present study is the first to clarify the mechanism of Cs+ resistance in certain bacteria. The study findings offer important insights into the mechanism of bacterial resistance to excess Cs+ in the environment, suggesting the potential for bioremediation in high Cs-contaminated areas.

Isolation and Cs+ resistance mechanism of Escherichia coli strain ZX-1

“…The variance in Cs + resistance observed among the E. coli strains in our study underscores the complex nature of Cs + tolerance mechanisms. The growth inhibition of strain Mach1™/pBAD24 at a Cs + concentration of 300 mM is consistent with previous observations of Cs + sensitivity in E. coli, as reported by Ishida et al (2023a) for E. coli W3110. However, the extraordinary resistance of strain ZX-1, which thrived at 800 mM Cs + , indicates its unique resistance capabilities, which merits further exploration.…”

“…Therefore, it is suggested that ribosomes may have overcome Cs + -induced destabilization. Notably, despite our previous report's minimal impact on ribosome complexes due to Cs + treatment, the Bacillus subtilis did not exhibit Cs + resistance (Ishida et al, 2023a). This suggests that the destabilization of ribosomes alone may not be the direct cause of cell death in bacteria exposed to Cs + .…”

“…Ribosomes were prepared and fractionated using the method of Ishida et al (2023a) . Mach1 ™ /pBAD24, ZX-1, and ΔZX-1/pBR322ΔAp were pre-cultured at 37°C for 16 h on an LB agar plate.…”

Physiological importance and role of Mg2+ in improving bacterial resistance to cesium

“…In addition, it is a high-concentration Cs + -resistant bacterium that can grow in media containing 1,200 mM CsCl ( Koretsune et al, 2022 ). Two significant mechanisms of Cs + resistance in TS-1 have been reported ( Koretsune et al, 2022 ; Ishida et al, 2023 ). The first mechanism works through the Cs + /H + antiporter CshA, which excretes Cs + from the cell and maintains a low intracellular Cs + concentration.…”

“…The Cs + -sensitive mutant strain Mut4 is a CshA defective mutant isolated previously ( Koretsune et al, 2022 ). The second mechanism involves maintaining Cs + resistance through the uptake of magnesium ions (Mg 2+ ) into cells via the magnesium transporter MgtE, even when the Cs + concentration increases ( Ishida et al, 2023 ). A mutation in the mgtE gene of TS-1 (mutant strain Mut5) leads to Cs + -sensitivity because of the deficiency of intracellular Mg 2+ ( Ishida et al, 2023 ), suggesting that Mg 2+ plays an essential role in Cs + resistance in TS-1.…”