Harnessing desiccation-tolerant cyanobacteria as biosensors for LexA-mediated responses in deep space

A LexA-mediated biosensor tailored for space applications was developed leveraging the desert cyanobacterium Chroococcidiopsis sp. CCMEE 029 engineered with a transcriptional fusion between an SOS-responsive promoter and a luciferase gene reporter.

The SOS DNA repair system was investigated by bioinformatics analysis that identified a lexA gene in the genome CCMEE 029 encoding a protein sharing key structural features with other cyanobacterial homologs. Consensus LexA-binding motifs upstream genes involved in DNA-damage repair, photosynthesis, and oxidative-stress defense were computationally identified and the DNA binding of CCMEE 029' LexA was confirmed by molecular modeling and molecular dynamics simulations. The expression of lexA and recA after treatment for 30 min with 10 mM H₂O₂ was evaluated by RT-qPCR. Then Chroococcidiopsis sp. CCMEE 029 was transformed with a plasmid carrying a transcriptional fusion between the upstream region of the recA gene containing the LexA-binding motif and a firefly luciferase gene. The biosensor responsiveness was tested in response to DNA-damaging agents, after desiccation as well as under simulated microgravity conditions combined with γ-rays.

Upon the addition of the luciferin substrate, Chroococcidiopsis transformants exposed to DNA-damaging agents (H₂O₂, ultra-violet C (UVC), and γ-ray irradiation) emitted a bioluminescent signal. A correlation was detected between increased UVC doses and the onset of detectable DNA damage. The biosensor responsiveness was confirmed under simulated microgravity conditions combined with γ-rays used as a proxy of the space environment. Notably, the biosensor responsiveness was retained after 4.5 months of air-dried storage as demonstrated by signal emission after rehydration and UVC and γ-ray irradiation.

Although a more global stress management role for LexA remains to be investigated in Chroococcidiopsis sp. CCMEE 029. The tested biosensor's feature supports a future integration of the air-dried biosensor into satellites and its in-orbit reactivation for real-time monitoring of the effects of space conditions, thus advancing future space exploration.