Molecular cloning and characterization of hetR genes from filamentous cyanobacteria

Structure of transcription factor HetR required for heterocyst differentiation in cyanobacteria

HetR is an essential regulator of heterocyst development in cyanobacteria. HetR binds to a DNA palindrome upstream of the hetP gene. We report the crystal structure of HetR from Fischerella at 3.0 Å. The protein is a dimer comprised of a central DNA-binding unit containing the N-terminal regions of the two subunits organized with two helix-turn-helix motifs; two globular flaps extending in opposite directions; and a hood over the central core formed from the C-terminal subdomains. The flaps and hood have no structural precedent in the protein database, therefore representing new folds. The structural assignments are supported by site-directed mutagenesis and DNA-binding studies. We suggest that HetR serves as a scaffold for assembly of transcription components critical for heterocyst development.

Improved eco-friendly recombinant Anabaena sp. strain PCC7120 with enhanced nitrogen biofertilizer potential

Photosynthetic, nitrogen-fixing Anabaena strains are native to tropical paddy fields and contribute to the carbon and nitrogen economy of such soils. Genetic engineering was employed to improve the nitrogen biofertilizer potential of Anabaena sp. strain PCC7120. Constitutive enhanced expression of an additional integrated copy of the hetR gene from a light-inducible promoter elevated HetR protein expression and enhanced functional heterocyst frequency in the recombinant strain. The recombinant strain displayed consistently higher nitrogenase activity than the wild-type strain and appeared to be in homeostasis with compatible modulation of photosynthesis and respiration. The enhanced combined nitrogen availability from the recombinant strain positively catered to the nitrogen demand of rice seedlings in short-term hydroponic experiments and supported better growth. The engineered strain is stable, eco-friendly, and useful for environmental application as nitrogen biofertilizer in paddy fields.

Variable selection pressures across lineages in Trichodesmium and related cyanobacteria based on the heterocyst differentiation protein gene hetR

Due to the irreversible inhibition of nitrogenase by O2, N2 fixation is incompatible with the oxygenic photosynthesis of cyanobacteria. These organisms have therefore evolved various strategies for growing diazotrophically. One group of N2-fixing cyanobacteria has specialized cells, heterocysts, which contain the nitrogenase, lack the oxygenic photosystem II, and are virtually anoxic inside as the result of respiratory activity and a thick glycolipid cell wall. The hetR gene encodes a serine protease which is thought to be involved in the regulation of heterocyst development and in DNA binding. Although hetR is also present in many non-heterocystous N2-fixing cyanobacteria, its function in these organisms is unknown. In this study, hetR sequences of the N2-fixing, non-heterocystous cyanobacterium Trichodesmium spp. and related genera were examined for signatures of selection. In parsimony- or distance-based hetR phylogenies, the filamentous non-heterocystous cyanobacteria Symploca sp. and Leptolyngbya sp. were closest to Trichodesmium sp. However, accommodating molecular attributes of hetR such as nucleotide frequencies and rate heterogeneity in phylogenetic analyses suggested that many other genera could not be excluded as sister taxa of Trichodesmium. Maximum likelihood analysis of the dN/dS ratio (omega) showed that-irrespective of the use of Symploca, Leptolyngbya, or more distant taxa as an outgroup-the lineage between an outgroup and Trichodesmium (omega1=0.02-0.05) and a lineage leading to Trichodesmium erythraeum (omega1=0.02) were under much stronger purifying selection than the other lineages in Trichodesmium (omega0=0.13-0.32). Although the results from the maximum likelihood analyses are most trustworthy because of codon usage bias in Trichodesmium, the results from a simpler tree-based McDonald-Kreitman test were in general agreement. Due to their quite different assumptions, the combination of these two methods of analysis circumvents multiple testing which, in general, is problematic when using branch models. Although the causal selective forces underlying the substitution patterns in hetR have not yet been identified, these findings parallel the variety of physiological, molecular, and behavioral differences in cyanobacteria related to N2 fixation. The heterogeneity of selection pressures in Trichodesmium is more surprising, because multiple adaptation mechanisms have not been described in this genus.

Heterocyst development in Anabaena

Many filamentous nitrogen-fixing cyanobacteria protect nitrogenase from oxygen in differentiated cells called heterocysts. Heterocyst development is controlled by the availability of nitrogen compounds in the environment and by intrinsic factors that regulate the frequency and pattern of heterocysts along vegetative cell filaments. Recent progress in understanding heterocyst development in these simple multicellular organisms includes demonstrating the role of 2-oxoglutarate in regulating the activity of the transcription factor NtcA, the identification of additional genes in the regulatory network, such as hetF, and the further characterization of previously identified genes and proteins, including DevR/HepK, hetR, hetN, patS and patB.

Diurnal expression of hetR and diazocyte development in the filamentous non-heterocystous cyanobacterium Trichodesmium erythraeum

The marine non-heterocystous cyanobacterium Trichodesmium fixes atmospheric N(2) aerobically in light. In situ immunolocalization/light microscopy of NifH revealed that lighter, non-granulated cell regions observed correspond to the nitrogenase-containing diazocyte clusters in Trichodesmium IMS101. The number of diazocyte clusters per trichome varied from 0 to 4 depending on trichome length. The constant percentage of diazocytes (approx. 15 %) in cultured strains and five natural populations suggests a developmentally regulated differentiation process. Real-time RT-PCR showed that ntcA, encoding the global nitrogen regulator in cyanobacteria, and hetR, the key regulatory gene in heterocyst differentiation, are both constitutively expressed during a 12 h/12 h light/dark cycle. hetR in addition showed a distinct peak in the dark (close to midnight) while nifH expression commenced 6-8 h later. The expression of all three genes was negatively affected by addition of ammonia. Some early heterocyst differentiation genes were also identified in the genome of Trichodesmium. The data suggest that hetR and ntcA may be required for development and function of diazocytes in Trichodesmium.

Nitrogen fixation and photosynthetic oxygen evolution in cyanobacteria

The biological reduction of N(2) is catalyzed by nitrogenase, which is irreversibly inhibited by molecular oxygen. Cyanobacteria are the only diazotrophs (nitrogen-fixing organisms) that produce oxygen as a by-product of the photosynthetic process, and which must negotiate the inevitable presence of molecular oxygen with an essentially anaerobic enzyme. In this review, we present an analysis of the geochemical conditions under which nitrogenase evolved and examine how the evolutionary history of the enzyme complex corresponds to the physiological, morphological, and developmental strategies for reducing damage by molecular oxygen. Our review highlights biogeochemical constraints on diazotrophic cyanobacteria in the contemporary world.