Global transcriptome analyses and regulatory mechanisms in Halothece sp. PCC 7418 exposed to abiotic stresses

Halotolerant species are of interest since they occur naturally in environments with excess toxic ions. The cyanobacterium Halothece sp. PCC 7418 (hereafter referred to as Halothece) exhibits remarkable halotolerance and was used to examine stress-responsive regulatory mechanisms. The effects of five different stimuli on Halothece transcriptomes were examined using RNA sequencing. In response to diverse stresses, there were both common and stress-specific transcriptional responses. A common upregulated gene set under all stresses consisted of nine differentially expressed genes (DEGs). We also found that osmotic stress elicited the largest set of DEGs. Salt- and osmotic-responsive regulatory mechanisms shared common pathways. DEGs that were upregulated under salt stress encoded proteins involved in photosynthesis and related machineries. Furthermore, DEGs encoding two-component system (TCS) factors, transcriptional factors, scaffolds for protein-protein interactions, transporters, protein turnover factors, and lipid biosynthesis enzymes were also identified under salt stress. Notably, one-carbon (1C) metabolism factors, glycine betaine (GB) synthesis enzymes, and GB transporters were upregulated under salt stress. Metabolic analyses revealed that GB accumulated under salt stress, while mycosporine-2-glycine (M2G) accumulated under salt or osmotic stress. None of the nutrient starvations induced GB nor M2G accumulation. These results suggested that GB and M2G are two osmoprotectants that contribute to halotolerance. Based on our results, we proposed regulatory mechanisms that are crucial for halotolerance, which are coordinated with the GB, M2G, 1C, amino acid, and central carbon interlinking metabolic pathways. 1C metabolism directly fulfills the high metabolite requirements for halotolerance together with the ancillary role of several metabolic pathways.

Automated summary

Halotolerant species naturally occur in environments with excess toxic ions, making them interesting for study. The cyanobacterium Halothece sp. PCC 7418 (Halothece) was used to explore stress-responsive regulatory mechanisms. RNA sequencing was performed to examine the effects of five different stimuli on Halothece transcriptomes. The study found both common and stress-specific transcriptional responses to diverse stresses. Osmotic stress elicited the largest set of differentially expressed genes (DEGs), while salt- and osmotic-responsive regulatory mechanisms shared common pathways. Salt stress specifically upregulated genes involved in photosynthesis, two-component systems, transcription, protein-protein interactions, and lipid biosynthesis. Glycine betaine (GB) synthesis enzymes and transporters were also upregulated under salt stress. GB and mycosporine-2-glycine (M2G) accumulation was observed under salt or osmotic stress.