The evolutionarily conserved HtrA is associated with stress tolerance and protein homeostasis in the halotolerant cyanobacterium Halothece sp. PCC7418

Molecular and biochemical characterization of a plant-like iota-class glutathione S-transferase from the halotolerant cyanobacterium Halothece sp. PCC7418

AIMS

This study identifies a unique glutathione S-transferase (GST) in extremophiles using genome, phylogeny, bioinformatics, functional characterization, and RNA sequencing analysis.

METHODS AND RESULTS

Five putative GSTs (H0647, H0729, H1478, H3557, and H3594) were identified in Halothece sp. PCC7418. Phylogenetic analysis suggested that H0647, H1478, H0729, H3557, and H3594 are distinct GST classes. Of these, H0729 was classified as an iota-class GST, encoding a high molecular mass GST protein with remarkable features. The protein secondary structure of H0729 revealed the presence of a glutaredoxin (Grx) Cys-Pro-Tyr-Cys (C-P-Y-C) motif that overlaps with the N-terminal domain and harbors a topology similar to the thioredoxin (Trx) fold. Interestingly, recombinant H0729 exhibited a high catalytic efficiency for both glutathione (GSH) and 1-chloro-2, 4-dinitrobenzene (CDNB), with catalytic efficiencies that were 155- and 32-fold higher, respectively, compared to recombinant H3557. Lastly, the Halothece gene expression profiles suggested that antioxidant and phase II detoxification encoding genes are crucial in response to salt stress.

CONCLUSION

Iota-class GST was identified in cyanobacteria. This GST exhibited a high catalytic efficiency toward xenobiotic substrates. Our findings shed light on a diversified evolution of GST in cyanobacteria and provide functional dynamics of the genes encoding the enzymatic antioxidant and detoxification systems under abiotic stresses.

Halotolerance mechanisms in salt‑tolerant cyanobacteria

Cyanobacteria are ubiquitously distributed in nature and are the most abundant photoautotrophs on Earth. Their long evolutionary history reveals that cyanobacteria have a remarkable capacity and strong adaptive tendencies to thrive in a variety of conditions. Thus, they can survive successfully, especially in harsh environmental conditions such as salty environments, high radiation, or extreme temperatures. Among others, salt stress because of excessive salt accumulation in salty environments, is the most common abiotic stress in nature and hampers agricultural growth and productivity worldwide. These detrimental effects point to the importance of understanding the molecular mechanisms underlying the salt stress response. While it is generally accepted that the stress response mechanism is a complex network, fewer efforts have been made to represent it as a network. Substantial evidence revealed that salt-tolerant cyanobacteria have evolved genomic specific mechanisms and high adaptability in response to environmental changes. For example, extended gene families and/or clusters of genes encoding proteins involved in the adaptation to high salinity have been collectively reported. This chapter focuses on recent advances and provides an overview of the molecular basis of halotolerance mechanisms in salt‑tolerant cyanobacteria as well as multiple regulatory pathways. We elaborate on the major protective mechanisms, molecular mechanisms associated with halotolerance, and the global transcriptional landscape to provide a gateway to uncover gene regulation principles. Both knowledge and omics approaches are utilized in this chapter to decipher the mechanistic insights into halotolerance. Collectively, this chapter would have a profound impact on providing a comprehensive understanding of halotolerance in salt‑tolerant cyanobacteria.

Global transcriptional and circadian regulation in a halotolerant cyanobacterium Halothece sp. PCC7418

Substantial evidence has been accumulated about the molecular basis underlying halotolerance; however, insights into the regulatory networks for relevant genes and mechanisms of their interplay remain elusive. Here, we present a comprehensive transcriptome investigation, using RNA sequencing, of specific metabolic pathways and networks in a halotolerant cyanobacterium, Halothece sp. PCC7418, including the circadian rhythm profile. Dissecting the transcriptome presented the intracellular regulation of gene expressions, which was linked with ion homeostasis, protein homeostasis, biosynthesis of compatible solutes, and signal transduction, for adaptations to high-salinity environments. The efficient production and distribution of energy were also implicated in this acclimation process. Furthermore, we found that high-salinity environments had a dramatic effect on the global transcriptional expression regulated by the circadian clock. Our findings can provide a comprehensive transcriptome for elucidating the molecular mechanisms underlying halotolerance in cyanobacteria.

Plant protease as regulator and signaling molecule for enhancing environmental stress-tolerance

Proteases are ubiquitous in prokaryotes and eukaryotes. Plant proteases are key regulators of various physiological processes, including protein homeostasis, organelle development, senescence, seed germination, protein processing, environmental stress response, and programmed cell death. Proteases are involved in the breakdown of peptide bonds resulting in irreversible posttranslational modification of the protein. Proteases act as signaling molecules that specifically regulate cellular function by cleaving and triggering receptor molecules. Peptides derived from proteolysis regulate ROS signaling under oxidative stress in the plant. It degrades misfolded and abnormal proteins into amino acids to repair the cell damage and regulates the biological process in response to environmental stress. Proteases modulate the biogenesis of phytohormones which control plant growth, development, and environmental stresses. Protein homeostasis, the overall balance between protein synthesis and proteolysis, is required for plant growth and development. Abiotic and biotic stresses are major factors that negatively impact cellular survivability, biomass production, and reduced crop yield potentials. Therefore, the identification of various stress-responsive proteases and their molecular functions may elucidate valuable information for the development of stress-resilient crops with higher yield potentials. However, the understanding of molecular mechanisms of plant protease remains unexplored. This review provides an overview of proteases related to development, signaling, and growth regulation to acclimatize environmental stress in plants.

Molecular and functional insights into glutathione S-transferase genes associated with salt stress in Halothece sp. PCC7418

Evolution and function of glutathione S-transferase (GST) in primordial oxygenic phototrophs such as cyanobacteria are poorly understood. In this study, we identified and functionally characterized the GST gene family in the halotolerant cyanobacterium Halothece sp. PCC7418. Four putative Halothece-GSTs had very low homology, which implies evolutionary divergence. Of these, H0647, H0729 and H3557 were differentially expressed by oxidative stress whereas H3557 was highly and specifically upregulated under salt stress. In vitro analysis revealed that the recombinant H3557 exhibited GST activity toward 1-chloro-2, 4-dinitrobenzene (CDNB) and glutathione (GSH). H3557 displayed a broad range of activity at pH 6.5-10.5. Kinetic parameters showed the apparent K_m for CDNB and GSH was 0.14 and 0.75 mM, respectively. H3557 remained catalytically active in the presence of NaCl. Structural modelling supported that H3557 is salt-adaptive enzyme with highly acidic residues on the protein surface. The vital function of H3557 in heterologous expression system was evaluated. The H3557-expressing cells were more tolerant to H₂ O₂ -induced oxidative stress compared with other GST-expressing cells and conferred salt tolerance. Taken together, the findings of this study provide insights into the molecular and cellular functions of GST in cyanobacteria, particularly under salt stress, which is less understood compared with other species.

Insights into the phylogeny and transcriptional response of serine proteases in a halotolerant cyanobacterium Halothece sp. PCC7418

Serine proteases are a class of versatile proteolytic enzymes. They are necessary for protein catabolism, intracellular amino acid turnover, and regulation of proteins involved in diverse molecular and cellular processes across taxa. In this study, bioinformatic analyses revealed a significantly large number of serine proteases in the halotolerant cyanobacterium Halothece sp. PCC7418 (hereafter referred to as Halothece 7418) compared to the model freshwater cyanobacterium Synechococcus elongatus PCC7942 (hereafter referred to as S. elongatus 7942). The cyanobacterial serine proteases are likely derived from different linages since no conserved motifs were detected. The presence of highly diverse serine proteases in Halothece 7418 implicated an evolutionary-mediated modification of several proteases, which may play numerous physiological roles. We also examined the gene expression patterns of 34 serine protease encoding genes in Halothece 7418 exposed to salt stress. Our results revealed that several serine protease genes were drastically up-regulated under salt with high concentration but remained unchanged under salt with low concentration. All four clp genes (H1996, H1997, H0950, and H3375) and H3553 gene (which encodes a putative HtrA protease) were significantly induced upon salt stress. These responses support the roles of the housekeeping pathways in both the degradation of damaged proteins induced by salt stress and regulation of proteins involved in the molecular recovery from salt stress. Since serine proteases share several biochemical features and physiological functions, the results from this study provide an insight into diversification of serine proteases in cyanobacteria. Further, these results will increase our understanding of several mechanisms at the subcellular level.