A single gene <Emphasis Type="Italic">all3940</Emphasis> (Dps) overexpression in <Emphasis Type="Italic">Anabaena</Emphasis> sp. PCC 7120 confers multiple abiotic stress tolerance via proteomic alterations

DNA-binding proteins (Dps) induced during starvation play an important role in gene regulation and maintaining homeostasis in bacteria. The nitrogen-fixing cyanobacterium, Anabaena PCC7120, has four genes annotated as coding for Dps; however, the information on their physiological roles is limiting. One of the genes coding for Dps, ‘all3940’ was found to be induced under different abiotic stresses in Anabaena and upon overexpression enhanced the tolerance of Anabaena to a multitude of stresses, which included salinity, heat, heavy metals, pesticide, and nutrient starvation. On the other hand, mutation in the gene resulted in decreased growth of Anabaena. The modulation in the levels of All3940 in Anabaena, achieved either by overexpression of the protein or mutation of the gene, resulted in changes in the proteome, which correlated well with the physiological changes observed. Proteins required for varied physiological activities, such as photosynthesis, carbon-metabolism, oxidative stress alleviation, exhibited change in protein profile upon modulation of All3940 levels in Anabaena. This suggested a direct or an indirect effect of All3940 on the expression of the above stress-responsive proteins, thereby enhancing tolerance in Anabaena PCC7120. Thus, All3940, though categorized as a Dps, is possibly a general stress protein having a global role in regulating tolerance to multitude of stresses in Anabaena.     

LexA protein of cyanobacterium Anabaena sp. strain PCC7120 exhibits in vitro pH-dependent and RecA-independent autoproteolytic activity

The LexA protein of the nitrogen-fixing cyanobacterium, Anabaena sp. strain PCC7120 exhibits a RecA-independent and alkaline pH-dependent autoproteolytic cleavage. The autoproteolytic cleavage of Anabaena LexA occurs at pH 8.5 and above, stimulated by the addition of Ca²⁺ and in the temperature range of 30–57 °C. Mutational analysis of Anabaena LexA protein indicated that the cleavage occurred at the peptide bond between Ala-84 and Gly-85, and optimal cleavage required the presence of Ser-118 and Lys-159, as also observed for LexA protein of Escherichia coli. Cleavage of Anabaena LexA was affected upon deletion of three amino acids, ⁸⁶GLI. These three amino acids are unique to all cyanobacterial LexA proteins predicted to be cleavable. The absence of RecA-dependent cleavage at physiological pH, which has not been reported for other bacterial LexA proteins, is possibly due to the absence of RecA interacting sites on Anabaena LexA protein, corresponding to the residues identified in E. coli LexA, and low cellular levels of RecA in Anabaena. Exposure to SOS-response inducing stresses, such as UV-B and mitomycin C neither affected the expression of LexA in Anabaena nor induced cleavage of LexA in either Anabaena 7120 or E. coli overexpressing Anabaena LexA protein. Though the LexA may be acting as a repressor by binding to the LexA box in the vicinity of the promoter region of specific gene, their derepression may not be via proteolytic cleavage during SOS-inducing stresses, unless the stress induces increase in cytoplasmic pH. This could account for the regulation of several carbon metabolism genes rather than DNA-repair genes under the regulation of LexA in cyanobacteria especially during high light induced oxidative stress.     

Characterization of two naturally truncated, Ssb-like proteins from the nitrogen-fixing cyanobacterium, Anabaena sp. PCC7120

Single-stranded (ss) DNA-binding (Ssb) proteins are vital for all DNA metabolic processes and are characterized by an N-terminal OB-fold followed by P/G-rich spacer region and a C-terminal tail. In the genome of the heterocystous, nitrogen-fixing cyanobacterium, Anabaena sp. strain PCC 7120, two genes alr0088 and alr7579 are annotated as ssb, but the corresponding proteins have only the N-terminal OB-fold and no P/G-rich region or acidic tail, thereby rendering them unable to interact with genome maintenance proteins. Both the proteins were expressed under normal growth conditions in Anabaena PCC7120 and regulated differentially under abiotic stresses which induce DNA damage, indicating that these are functional genes. Constitutive overexpression of Alr0088 in Anabaena enhanced the tolerance to DNA-damaging stresses which caused formation of DNA adducts such as UV and MitomycinC, but significantly decreased the tolerance to γ-irradiation, which causes single- and double-stranded DNA breaks. On the other hand, overexpression of Alr7579 had no significant effect on normal growth or stress tolerance of Anabaena. Thus, of the two truncated Ssb-like proteins, Alr0088 may be involved in protection of ssDNA from damage, but due to the absence of acidic tail, it may not aid in repair of damaged DNA. These two proteins are present across cyanobacterial genera and unique to them. These initial studies pave the way to the understanding of DNA repair in cyanobacteria, which is not very well documented.     

The SbcC and SbcD homologs of the cyanobacterium <Emphasis Type="Italic">Anabaena</Emphasis> sp. strain PCC7120 (Alr3988 and All4463) contribute independently to DNA repair

The ubiquitous SbcCD exonuclease complex has been shown to perform an important role in DNA repair across prokaryotes and eukaryotes. However, they have remained uncharacterized in the ancient and stress-tolerant cyanobacteria. In the cyanobacterium Anabaena sp. strain PCC7120, SbcC and SbcD homologs, defined on the basis of the presence of corresponding functional domains, are annotated as hypothetical proteins, namely Alr3988 and All4463 respectively. Unlike the presence of sbcC and sbcD genes in a bicistronic operon in most organisms, these genes were distantly placed on the chromosome in Anabaena, and found to be negatively regulated by LexA. Both the genes were found to be essential in Anabaena as the individual deletion mutants were non-viable. On the other hand, the proteins could be individually overexpressed in Anabaena with no effect on normal cell physiology. However, they contributed positively to enhance the tolerance to different DNA damage-inducing stresses, such as mitomycin C and UV- and γ-radiation. This indicated that the two proteins, at least when overexpressed, could function independently and mitigate the damage caused due to the formation of DNA adducts and single- and double-strand breaks in Anabaena. This is the first report on possible independent in vivo functioning of SbcC and SbcD homologs in any bacteria, and the first effort to functionally characterize the proteins in any cyanobacteria.     

植物microRNA对重金属胁迫响应的调控

镉是重要的重金属污染物,它严重影响植物生长,并危害人体健康.植物的重金属耐受机理很复杂,需从多个角度阐述.但参与重金属应答的调节性基因研究较少,重金属响应调控网络并不清楚.microRNA(miRNA)是一类新型的调控基因表达的     

植物盐胁迫应答转录因子的研究进展

盐胁迫会导致植物受到初级的渗透胁迫和离子毒害以及次级的氧化胁迫和营养胁迫,严重制约了农业生产.植物盐胁迫应答转录因子能够通过调节下游靶基因的表达减轻盐胁迫对植物造成的伤害.文中基于土壤盐渍化及其对植物的危害、转录因子在植物盐胁迫信号转导网络中的中枢调节作用,综述了盐胁迫应答转录因子参与的盐胁迫信号转导途径、通过形成同源...     

Expression of ethylene response factor JERF1 in rice improves tolerance to drought

Ethylene response factor (ERF) proteins regulate a variety of stress responses in plant. JERF1, a tomato ERF protein, can be induced by abscisic acid (ABA). Overexpression of JERF1 enhanced the tolerance of transgenic tobacco to high salt concentration, osmotic stress, and low temperature by regulating the expression of stress-responsive genes by binding to DRE/CRT and GCC-box cis-elements. In this research, we further report that overexpression of JERF1 significantly enhanced drought tolerance of transgenic rice. The overexpression activated the expression of stress-responsive genes and increased the synthesis of the osmolyte proline by regulating the expression of OsP5CS, encoding the proline biosynthesis key enzyme deltal-pyrroline-5-carboxylate synthetase. JERF1 also activated the expression of two ABA biosynthesis key enzyme genes, OsABA2 and Os03g0810800, and increased the synthesis of ABA in rice. Analysis of cis-elements of JERF1-targeted genes pointed to the existence of DRE/CRT and/or GCC box in their promoters, indicating that JERF1 could activate the expression of related genes in rice by binding to these cis-elements. Unlike some other ERF proteins, constructive overexpression of JERF1 did not change the growth and development of transgenic rice, which makes JEFR1 a potentially useful source in breeding for greater tolerance to abiotic stress.     

Role of Phytohormones in Regulating Heat Stress Acclimation in Agricultural Crops

Heat stress (HS) seriously affects crop growth, causing significant crop yield losses worldwide. The regulatory mechanisms controlling HS tolerance in plants are not well understood. Phytohormones are important molecules for coordinating myriad of phenomena related to plant growth and development. They are also essential endogenous signaling molecules that actively mediate numerous physiological responses under abiotic stress by triggering stress-responsive regulatory genes involved in plant growth. This review updates the central role of various phytohormones—indole acetic acid, gibberellic acid, abscisic acid, cytokinins, ethylene, salicylic acid, brassinosteroids, strigolactone, and jasmonic acid—in regulating the HS response so that plants can adapt to increasing temperature stress. We also reveal how these stress-responsive phytohormones switch on various regulatory gene(s) and genes encoding antioxidants and heat shock proteins (HSPs) to combat HS in various plant species.

     

The response regulator RpaB binds the high light regulatory 1 sequence upstream of the high-light-inducible <Emphasis Type="Italic">hliB</Emphasis> gene from the cyanobacterium <Emphasis Type="Italic">Synechocystis</Emphasis> PCC 6803

Cyanobacteria, like other photosynthetic organisms, respond to the potentially damaging effects of high-intensity light by regulating the expression of a variety of stress-responsive genes through regulatory mechanisms that remain poorly understood. The high light regulatory 1 (HLR1) sequence can be found upstream of many genes regulated by high-light (HL) stress in cyanobacteria. In this study, we identify the factor that binds the HLR1 upstream of the HL-inducible hliB gene in the cyanobacterium Synechocystis PCC 6803 as the RpaB (Slr0947) response regulator.     

Differential regulation of ssb genes in the nitrogen‐fixing cyanobacterium,Anabaena sp. strain PCC71201

Anabaena sp. PCC7120 possesses three genes coding for single‐stranded DNA‐binding (SSB) protein, of which ssb1 was a single gene, and ssb2 and ssb3 are the first genes of their corresponding operons. Regulation of the truncated ssb genes, ssb1 (alr0088) and ssb2 (alr7559), was unaffected by N‐status of growth. They were negatively regulated by the SOS‐response regulatory protein LexA, as indicated by the (i) binding of Anabaena LexA to the LexA box of regulatory regions of ssb1 and ssb2, and (ii) decreased expression of the downstream gfp reporter gene in Escherichia coli upon co‐expression of LexA. However, the full‐length ssb gene, ssb3 (all4779), was regulated by the availability of Fe²⁺ and combined nitrogen, as indicated by (i) increase in the levels of SSB3 protein on Fe²⁺‐depletion and decrease under Fe²⁺‐excess conditions, and (ii) 1.5‐ to 1.6‐fold decrease in activity under nitrogen‐fixing conditions compared to nitrogen‐supplemented conditions. The requirement of Fe²⁺ as a co‐factor for repression by FurA and the increase in levels of FurA under nitrogen‐deficient conditions in Anabaena (Lopez‐Gomollon et al. 2007) indicated a possible regulation of ssb3 by FurA. This was substantiated by (i) the binding of FurA to the regulatory region of ssb3, (ii) repression of the expression of the downstream gfp reporter gene in E. coli upon co‐expression of FurA, and (iii) negative regulation of ssb3 promoter activity by the upstream AT‐rich region in Anabaena. This is the first report on possible role of FurA, an important protein for iron homeostasis, in DNA repair of cyanobacteria.