Unusually wide co-factor tolerance in a metalloenzyme; divalent metal ions modulate endo-exonuclease activity in T5 exonuclease

TLTC, a T5 exonuclease-mediated low-temperature DNA cloning method

Molecular cloning is used in a wide variety of biological and medical research. Here, we developed a rapid and efficient DNA-assembling method for routine laboratory work. We discovered that the cleavage speed of T5 exonuclease is approximately 3 nt/min at 0°C and hence developed a T5 exonuclease-mediated low-temperature sequence- and ligation-independent cloning method (TLTC). Two homologous regions of 15 bp-25 bp compatible with the ends of the vector backbones were introduced into the inserts through PCR. Approximately 120 fmol of inserts and linear vectors was mixed at a molar ratio of approximately 3:1 and treated with 0.5 U of T5 exonuclease at 0°C for 5 min. Then, the mixture was transformed into Escherichia coli to generate recombinant plasmids. Single segment and multi-segments can be assembled efficiently using TLTC. For single segment, the overall cloning efficiency is above 95%. Moreover, extra nucleotides in the vectors can be removed during TLTC. In conclusion, an extremely simple and fast DNA cloning/assembling method was established in the present study. This method facilitates routine DNA cloning and synthesis of DNA fragments.

Activity of MCPIP1 RNase in tumor associated processes

The monocyte chemoattractant protein-induced protein (MCPIP) family consists of 4 members (MCPIP1–4) encoded by the ZC3h12A-D genes, which are located at different loci. The common features of MCPIP proteins are the zinc finger domain, consisting of three cysteines and one histidine (CCCH), and the N-terminal domain of the PilT protein (PilT-N-terminal domain (PIN domain)). All family members act as endonucleases controlling the half-life of mRNA and microRNA (miRNA). The best-studied member of this family is MCPIP1 (also known as Regnase-1).

In this review, we discuss the current knowledge on the role of MCPIP1 in cancer-related processes. Because the characteristics of MCPIP1 as a fundamental negative regulator of immune processes have been comprehensively described in numerous studies, we focus on the function of MCPIP1 in modulating apoptosis, angiogenesis and metastasis.

Comprehensive classification of the PIN domain-like superfamily

PIN-like domains constitute a widespread superfamily of nucleases, diverse in terms of the reaction mechanism, substrate specificity, biological function and taxonomic distribution. Proteins with PIN-like domains are involved in central cellular processes, such as DNA replication and repair, mRNA degradation, transcription regulation and ncRNA maturation. In this work, we identify and classify the most complete set of PIN-like domains to provide the first comprehensive analysis of sequence–structure–function relationships within the whole PIN domain-like superfamily. Transitive sequence searches using highly sensitive methods for remote homology detection led to the identification of several new families, including representatives of Pfam (DUF1308, DUF4935) and CDD (COG2454), and 23 other families not classified in the public domain databases. Further sequence clustering revealed relationships between individual sequence clusters and showed heterogeneity within some families, suggesting a possible functional divergence. With five structural groups, 70 defined clusters, over 100,000 proteins, and broad biological functions, the PIN domain-like superfamily constitutes one of the largest and most diverse nuclease superfamilies. Detailed analyses of sequences and structures, domain architectures, and genomic contexts allowed us to predict biological function of several new families, including new toxin-antitoxin components, proteins involved in tRNA/rRNA maturation and transcription/translation regulation.

Archaeal genome guardians give insights into eukaryotic DNA replication and damage response proteins

As the third domain of life, archaea, like the eukarya and bacteria, must have robust DNA replication and repair complexes to ensure genome fidelity. Archaea moreover display a breadth of unique habitats and characteristics, and structural biologists increasingly appreciate these features. As archaea include extremophiles that can withstand diverse environmental stresses, they provide fundamental systems for understanding enzymes and pathways critical to genome integrity and stress responses. Such archaeal extremophiles provide critical data on the periodic table for life as well as on the biochemical, geochemical, and physical limitations to adaptive strategies allowing organisms to thrive under environmental stress relevant to determining the boundaries for life as we know it. Specifically, archaeal enzyme structures have informed the architecture and mechanisms of key DNA repair proteins and complexes. With added abilities to temperature-trap flexible complexes and reveal core domains of transient and dynamic complexes, these structures provide insights into mechanisms of maintaining genome integrity despite extreme environmental stress. The DNA damage response protein structures noted in this review therefore inform the basis for genome integrity in the face of environmental stress, with implications for all domains of life as well as for biomanufacturing, astrobiology, and medicine.

Does life consistently maximise energy intensity?

We propose that a basic biological imperative of all organisms is to maximise energy (E) intensity, defined as the average rate of energy use per unit area of the Earth's surface. The dominant organism in any given environment is predicted to be that exerting the greatest E intensity regardless of evolutionary origin. Our 'theory of biological E intensity' thus explains variation in life form in terms of adaptations as opposed to accidents of biological history. It defines the competitive criterion in all metabolic pathways and industrial processes as the average rate of kinetic energy use, excluding heating but including all directed biological kinesis at scales up to the whole organism. A suggested unit for E intensity is joules per square meter per year. Because catalysts are crucial to extremely rapid use of energy (and therefore maximisation of E intensity), catalytic nutrient elements can be viewed as the ultimate means of life. It follows that a common denominator of all dominant organisms would be the acquisition of an optimal catalytic formula as determined by concentrations and ratios of C, H, O, N, S, Na, Mg, P, K, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Se, Mo, Cd, I, W, and Hg. The likely local shortages of various of these elements can theoretically be alleviated by various changes in the size, shape, and/or behaviour of organisms, depending on the environment. Thus, the availability, and potential for supplementation, of catalytic elements would be the ultimate basis for adaptation, largely determining which life form dominates in any particular location. The theory predicts the following. (1) In nutrient-rich environments offering the optimal catalytic formula, dominant organisms will be microbes. This is because microbes, and prokaryotes in particular, excel in E intensity through rapid biomolecular turnover, enabling them to usurp resources despite minimising biomass, complexity, and information. (2) Where the environment is catabolically dystrophic (i.e. scarce in certain nutrients required for catabolism), macrobes (e.g. humans and trees) will be superior competitors because they can collect and supplement nutrients and thereby approach the optimal catalytic formula. This enables macrobes, despite having considerably slower metabolism per unit body mass, to enhance E intensity relative to competing microbes constrained by catabolic dystrophy. Finally, (3) where the environment is anabolically dystrophic (i.e. scarce in certain nutrients required for anabolism) microbes will again dominate because biomolecular turnover can be relatively free from constraint given the limited fuel available. We suggest that an important and overlooked way to achieve power is to reuse energy, and that all organisms maximise E intensity by converting chemical potential energy (i.e. in fuel) into circuits of electromagnetic energy comprising electric charge, photons, and excited electrons. Because space and time merge subatomically, these electromagnetic circuits represent a concentration in spacetime of energy that (1) is concurrently kinetic and static, hence available for immediate use yet also conserved with minimal dissipation, and (2) ultimately promotes catalysis, which we assert is the primary biological tactic for maximising E intensity and thus fitness.

Dynamic evidence for metal ion catalysis in the reaction mediated by a flap endonuclease

On the basis of structural work, metal ions are proposed to play a catalytic role in reactions mediated by many phosphoryl transfer enzymes. To gain dynamic support for such mechanisms, the role of metal ion cofactors in phosphate diester hydrolysis catalysed by a flap endonuclease has been studied. The pH maximal rate profiles were measured in the presence of various metal ion cofactors; in each case, a single ionic form of the enzyme/cofactor accounts for the pH dependence. The kinetic pK(a)s display good correlation with the acidity of the corresponding hexahydrated metal ions, which strongly suggests a role for metal-bound hydroxide, or its equivalent ionic species, in the reaction. Comparing rates of reaction in the pH-independent regions, a small negative beta(nuc) value is observed. This suggests that expected trends in the nucleophilicity of the various metal-bound hydroxides are balanced by a second form of metal ion catalysis that is related to the acidity of the hexahydrated metal ion. This is likely to be either electrophilic catalysis or leaving group activation.

Interactions of mutant and wild-type flap endonucleases with oligonucleotide substrates suggest an alternative model of DNA binding

Previous structural studies on native T5 5' nuclease, a member of the flap endonuclease family of structure-specific nucleases, demonstrated that this enzyme possesses an unusual helical arch mounted on the enzyme's active site. Based on this structure, the protein's surface charge distribution, and biochemical analyses, a model of DNA binding was proposed in which single-stranded DNA threads through the archway. We investigated the kinetic and substrate-binding characteristics of wild-type and mutant nucleases in relation to the proposed model. Five basic residues R33, K215, K241, R172, and R216, are all implicated in binding branched DNA substrates. All these residues except R172 are involved in binding to duplex DNA carrying a 5' overhang. Replacement of either K215 or R216 with a neutral amino acid did not alter kcat appreciably. However, these mutant nucleases displayed significantly increased values for Kd and Km. A comparison of flap endonuclease binding to pseudoY substrates and duplexes with a single-stranded 5' overhang suggests a better model for 5' nuclease-DNA binding. We propose a major revision to the binding model consistent with these biophysical data.

Cleavage specificity of Saccharomyces cerevisiae flap endonuclease 1 suggests a double-flap structure as the cellular substrate

Flap endonuclease 1 (FEN1) is a structure-specific nuclease that cleaves substrates containing unannealed 5'-flaps during Okazaki fragment processing. Cleavage removes the flap at or near the point of annealing. The preferred substrate for archaeal FEN1 or the 5'-nuclease domains of bacterial DNA polymerases is a double-flap structure containing a 3'-tail on the upstream primer adjacent to the 5'-flap. We report that FEN1 in Saccharomyces cerevisiae (Rad27p) exhibits a similar specificity. Cleavage was most efficient when the upstream primer contained a 1-nucleotide 3'-tail as compared with the fully annealed upstream primer traditionally tested. The site of cleavage was exclusively at a position one nucleotide into the annealed region, allowing human DNA ligase I to seal all resulting nicks. In contrast, a portion of the products from traditional flap substrates is not ligated. The 3'-OH of the upstream primer is not critical for double-flap recognition, because Rad27p is tolerant of modifications. However, the positioning of the 3'-nucleotide defines the site of cleavage. We have tested substrates having complementary tails that equilibrate to many structures by branch migration. FEN1 only cleaved those containing a 1-nucleotide 3'-tail. Equilibrating substrates containing 12-ribonucleotides at the end of the 5'-flap simulates the situation in vivo. Rad27p cleaves this substrate in the expected 1-nucleotide 3'-tail configuration. Overall, these results suggest that the double-flap substrate is formed and cleaved during eukaryotic DNA replication in vivo.