RNA ligands to the tachykinin substance P have been selected from a large pool of random sequence RNA molecules. Substance P is an undecapeptide that plays a variety of roles as a neurotransmitter and neuromodulator in the central and peripheral nervous system of mammals. A systematic evolution of ligands by exponential enrichment (SELEX) procedure was used to isolate RNAs that bind substance P immobilized on a solid support. RNAs that also bind substance P in solution were identified, and the tightest binder was subjected to mutagenesis in a second SELEX procedure to evolve ligands with a higher affinity for the peptide. A comparative analysis of 36 ligands isolated from the second SELEX experiment revealed two main sequence classes with highly conserved secondary structures within each class. Dissociation constants for the interaction of these ligands with substance P in solution were determined by equilibrium dialysis. The amino acid residues involved in the interaction with the highest affinity ligand (190 nM Kd) were mapped by determining which of a set of overlapping fragments of substance P can compete with the intact peptide for binding. A binding competition experiment also demonstrated the ability of the same ligand to discriminate between substance P and the reverse orientation of the same amino acid sequence. The results from this study demonstrate that SELEX can yield high affinity RNA ligands to small nonconstrained peptides.

BackgroundHydrogen photo-production in green algae, catalyzed by the enzyme [FeFe]-hydrogenase (HydA), is considered a promising source of renewable clean energy. Yet, a significant increase in hydrogen production efficiency is necessary for industrial scale-up. We have previously shown that a major challenge to be resolved is the inferior competitiveness of HydA with NADPH production, catalyzed by ferredoxin-NADP+-reductase (FNR). In this work, we explored the in vivo hydrogen production efficiency of Fd-HydA, where the electron donor ferredoxin (Fd) is fused to HydA and expressed in the model organism Chlamydomonas reinhardtii.ResultsWe show that once the Fd-HydA fusion gene is expressed in micro-algal cells of C. reinhardtii, the fusion enzyme is able to intercept photosynthetic electrons and use them for efficient hydrogen production, thus supporting the previous observations made in vitro. We found that Fd-HydA has a ~4.5-fold greater photosynthetic hydrogen production rate standardized for hydrogenase amount (PHPRH) than that of the native HydA in vivo. Furthermore, we provide evidence suggesting that the fusion protein is more resistant to oxygen than the native HydA.ConclusionsThe in vivo photosynthetic activity of the Fd-HydA enzyme surpasses that of the native HydA and shows higher oxygen tolerance. Therefore, our results provide a solid platform for further engineering efforts towards efficient hydrogen production in microalgae through the expression of synthetic enzymes.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0601-3) contains supplementary material, which is available to authorized users.

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A number of species of microalgae and cyanobacteria photosynthetically produce H2 gas by coupling water oxidation with the reduction of protons to molecular hydrogen, generating renewable energy from sunlight and water. Photosynthetic H2 production, however, is transitory, and there is considerable interest in increasing and extending it for commercial applications. Here we report a Petri-plate version of our previous, microplate-based assay that detects photosynthetic H2 production by algae. The assay consists of an agar overlay of H2 -sensing Rhodobacter capsulatus bacteria carrying a green fluorescent protein that responds to H2 produced by single algal colonies in the bottom agar layer. The assay distinguishes between algal strains that photoproduce H2 at different levels under high light intensities, and it does so in a simple, inexpensive, and high-throughput manner. The assay will be useful for screening both natural populations and mutant libraries for strains having increased H2 production, and useful for identifying various genetic factors that physiologically or genetically alter algal hydrogen production.

Genome inspection revealed nine putative heme‐binding, FixL‐homologous proteins in Chlamydomonas reinhardtii. The heme‐binding domains from two of these proteins, FXL1 and FXL5 were cloned, expressed in Escherichia coli, purified and characterized. The recombinant FXL1 and FXL5 domains stained positively for heme, while mutations in the putative ligand‐binding histidine FXL1‐H200S and FXL5‐H200S resulted in loss of heme binding. The FXL1 and FXL5 [Fe(II), bound O2] had Soret absorption maxima around 415 nm, and weaker absorptions at longer wavelengths, in concurrence with the literature. Ligand‐binding measurements showed that FXL1 and FXL5 bind O2 with moderate affinity, 135 and 222 μM, respectively. This suggests that Chlamydomonas may use the FXL proteins in O2‐sensing mechanisms analogous to that reported in nitrogen‐fixing bacteria to regulate gene expression.