Regulation of multiple abiotic stress tolerance by LexA in the cyanobacterium Anabaena sp. strain PCC7120

Interlink between ExoD (Alr2882), exopolysaccharide synthesis and metal tolerance in Nostoc sp. strain PCC 7120: Insight into its role, paralogs and evolution

Exopolysaccharides (EPS) produced by bacterial species are an important component of bacteria's survival strategy. Synthesis of EPS, principal component of extracellular polymeric substance, occurs through multiple pathways involving multitude of genes. While stress-induced concomitant increase in exoD transcript levels and EPS content have been shown earlier, experimental evidence for direct correlation is lacking. In the present study, role of ExoD in Nostoc sp. strain PCC 7120 was evaluated by generating a recombinant Nostoc strain AnexoD⁺, wherein the ExoD (Alr2882) protein was constitutively overexpressed. AnexoD⁺ exhibited higher EPS production, propensity for formation of biofilms and tolerance to Cd stress compared to vector control AnpAM cells. Both Alr2882 and its paralog All1787 exhibited 5 transmembrane domains, with only All1787 predicted to interact with several proteins in polysaccharide synthesis. Phylogenetic analysis of orthologs of these proteins across cyanobacteria indicated that the two paralogs Alr2882 and All1787 and their corresponding orthologs arose divergently during evolution, and could have distinct roles to perform in the biosynthesis of EPS. This study has thrown open the possibility of engineering overproduction of EPS and inducing biofilm formation through genetic manipulation of EPS biosynthesis genes in cyanobacteria, thus building a cost-effective green platform for large scale production of EPS.

Cd-induced cytosolic proteome changes in the cyanobacterium Anabaena sp. PCC7120 are mediated by LexA as one of the regulatory proteins

LexA, a well-characterized transcriptional repressor of SOS genes in heterotrophic bacteria, has been shown to regulate diverse genes in cyanobacteria. An earlier study showed that LexA overexpression in a cyanobacterium, Anabaena sp. PCC7120 reduces its tolerance to Cd stress. This was later shown to be due to modulation of photosynthetic redox poising by LexA under Cd stress. However, due to the global regulatory nature of LexA and the prior prediction of AnLexA-box in a few heavy metal-responsive genes, we speculated that LexA has a broad role in Cd tolerance, with regulation over a variety of Cd stress-responsive genes in addition to photosynthetic genes. Thus, to further expand the knowledge on the regulatory role of LexA in Cd stress tolerance, a cytosolic proteome profiling of Anabaena constitutively overexpressing LexA upon Cd stress was performed. The proteomic study revealed 25 differentially accumulated proteins (DAPs) in response to the combined effect of LexA overexpression and Cd stress, and the other 11 DAPs exclusively in response to either LexA overexpression or Cd stress. The 36 identified proteins were related with a variety of functions, including photosynthesis, C-metabolism, antioxidants, protein turnover, post-transcriptional modifications, and a few unknown and hypothetical proteins. The regulation of LexA on corresponding genes, and six previously reported Cd efflux transporters, was further validated by the presence of AnLexA-boxes, transcript, and/or promoter analyses. In a nutshell, this study identifies the regulation of Anabaena LexA on several Cd stress-responsive genes of various functions, hence expanding the regulatory role of LexA under Cd stress.

NtcA, LexA and heptamer repeats involved in the multifaceted regulation of DNA repair genes recF, recO and recR in the cyanobacterium Nostoc PCC7120

Regulation of DNA repair genes in cyanobacteria is an unexplored field despite some of them exhibiting high radio-resistance. With RecF pathway speculated to be the major double strand break repair pathway in Nostoc sp. strain PCC7120, regulation of recF, recO and recR genes was investigated. Bioinformatic approach-based identification of promoter and regulatory elements was validated using qRT-PCR analysis, reporter gene and DNA binding assays. Different deletion constructs of the upstream regulatory regions of these genes were analysed in host Nostoc as well as heterologous system Escherichia coli. Studies revealed: (1) Positive regulation of all three genes by NtcA, (2) Negative regulation by LexA, (3) Involvement of contiguous heptamer repeats with/without its yet to be identified interacting partner in regulating (i) binding of NtcA and LexA to recO promoter and its translation, (ii) transcription or translation of recF, (4) Translational regulation of recF and recO through non-canonical and distant S.D. sequence and of recR through a rare initiation codon. Presence of NtcA either precludes binding of LexA to AnLexA-Box or negates its repressive action resulting in higher expression of these genes under nitrogen-fixing conditions in Nostoc. Thus, in Nostoc, expression of recF, recO and recR genes is intricately regulated through multiple regulatory elements/proteins. Contiguous heptamer repeats present across the Nostoc genome in the vicinity of start codon or promoter is likely to have a global regulatory role. This is the first report detailing regulation of DSB repair genes in any algae.

Gamma (γ)-radiation stress response of the cyanobacterium Anabaena sp. PCC7120: Regulatory role of LexA and photophysiological changes

High radioresistance of the cyanobacterium, Anabaena sp. PCC7120 has been attributed to efficient DNA repair, protein recycling, and oxidative stress management. However, the regulatory network involved in these batteries of responses remains unexplored. In the present study, the role of a global regulator, LexA in modulating gamma (γ)-radiation stress response of Anabaena was investigated. Comparison of the cytosolic proteome profiles upon γ-radiation in recombinant Anabaena strains, AnpAM (vector-control) and AnlexA⁺ (LexA-overexpressing), revealed 41 differentially accumulated proteins, corresponding to 29 distinct proteins. LexA was found to be involved in the regulation of 27 of the corresponding genes based on the presence of AnLexA-Box, EMSA, and/or qRT-PCR studies. The majority of the regulated genes were found to be involved in C-assimilation either through photosynthesis or C-catabolism and oxidative stress alleviation. Photosynthesis, measured in terms of PSII photophysiological parameters and thylakoid membrane proteome was found to be affected by γ-radiation in both AnpAM and AnlexA⁺ cells, with LexA affecting them even under control growth conditions. Thus, LexA functioned as one of the transcriptional regulators involved in modulating γ-radiation stress response in Anabaena. This study could pave the way for a deeper understanding of the regulation of γ-radiation-responsive genes in cyanobacteria at large.

Cell Death in Cyanobacteria: Current Understanding and Recommendations for a Consensus on Its Nomenclature

Cyanobacteria are globally widespread photosynthetic prokaryotes and are major contributors to global biogeochemical cycles. One of the most critical processes determining cyanobacterial eco-physiology is cellular death. Evidence supports the existence of controlled cellular demise in cyanobacteria, and various forms of cell death have been described as a response to biotic and abiotic stresses. However, cell death research in this phylogenetic group is a relatively young field and understanding of the underlying mechanisms and molecular machinery underpinning this fundamental process remains largely elusive. Furthermore, no systematic classification of modes of cell death has yet been established for cyanobacteria. In this work, we analyzed the state of knowledge in the field of cyanobacterial cell death. Based on that, we propose unified criterion for the definition of accidental, regulated, and programmed forms of cell death in cyanobacteria based on molecular, biochemical, and morphologic aspects following the directions of the Nomenclature Committee on Cell Death (NCCD). With this, we aim to provide a guide to standardize the nomenclature related to this topic in a precise and consistent manner, which will facilitate further ecological, evolutionary, and applied research in the field of cyanobacterial cell death.

The role of transcriptional repressor activity of LexA in salt-stress responses of the cyanobacterium Synechocystis sp. PCC 6803

Different from typical LexA repressors in heterotrophic bacteria exerting SOS response by auto-cleavage, cyanobacterial LexAs, especially that of Synechocystis sp. PCC 6803 (S.6803), have been suggested be involved in regulation of a number of genes related to various cellular processes, rather than the typical SOS regulon. When and how cyanobacterial LexAs are triggered to regulate its target genes have remained unknown. In this study, we found the profound repressing effect of LexA on salt-stress inducible genes in S.6803. The repressing activity of LexA was likely to persist during salt stress and the salt response of these genes was mainly achieved by other regulators than LexA, suggesting that the physiological role of LexA is fine-tuning of gene expression in response to environmental changes. Although the amount and oligomeric state of LexA were unchanged upon salt stress, two-dimensional electrophoresis and liquid chromatography-tandem mass spectrometry analyses detected a change in posttranslational modification in a small fraction of LexA molecules, possibly dephosphorylation of Ser¹⁷³, after 30 min upon the upshift in salt concentration. Activity of LexA in S.6803 may be under gradual control by posttranslational modification to fine-tune gene expression, which is contrasted with the digital switching-off regulation by auto-cleavage in heterotrophic bacteria.

Construction of Antisense RNA-mediated Gene Knock-downStrains in the Cyanobacterium Anabaena sp. PCC 7120

Anabaena sp. PCC 7120 (hereafter Anabaena) is a model cyanobacterium to study nitrogen fixation, cellular differentiation and several other key biological functions that are analogous in plants. As with any other organism, many genes in Anabaena encode an essential life function and hence cannot be deleted, causing a bottleneck in the elucidation of its genomic function. Antisense RNA (asRNA) mediated approach renders the study of essential genes possible by suppressing (but not completely eliminating) expression of the target gene, thus allowing them to function to some extent. Recently, we have successfully implemented this approach using the strong endogenous promoter of the psbA1 gene (D1 subunit of Photosystem II) introduced into a high-copy replicative plasmid (pAM1956) to suppress the transcript level of the target gene alr0277 (encoding a sigma factor, SigJ/Alr0277) in Anabaena. This protocol represents an efficient and easy procedure to further explore the functional genomics, expanding the scope of basic and applied research in these ecologically important cyanobacteria.