TimiRGeN: R/Bioconductor package for time series microRNA-mRNA integration and analysis.

MOTIVATION: The analysis of longitudinal datasets and construction of gene regulatory networks (GRNs) provide a valuable means to disentangle the complexity of microRNA (miRNA)-mRNA interactions. However, there are no computational tools that can integrate, conduct functional analysis and generate detailed networks from longitudinal miRNA-mRNA datasets. RESULTS: We present TimiRGeN, an R package that uses time point-based differential expression results to identify miRNA-mRNA interactions influencing signaling pathways of interest. miRNA-mRNA interactions can be visualized in R or exported to PathVisio or Cytoscape. The output can be used for hypothesis generation and directing in vitro or further in silico work such as GRN construction. AVAILABILITY AND IMPLEMENTATION: TimiRGeN is available for download on Bioconductor (https://bioconductor.org/packages/TimiRGeN) and requires R v4.0.2 or newer and BiocManager v3.12 or newer. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.

Custom-designed zinc finger nucleases: what is next?

Custom-designed zinc finger nucleases (ZFNs)--proteins designed to cut at specific DNA sequences--combine the non-specific cleavage domain (N) of Fok I restriction endonuclease with zinc finger proteins (ZFPs). Because the recognition specificities of the ZFPs can be easily manipulated experimentally, ZFNs offer a general way to deliver a targeted site-specific double-strand break (DSB) to the genome. They have become powerful tools for enhancing gene targeting--the process of replacing a gene within a genome of cells via homologous recombination (HR)--by several orders of magnitude. ZFN-mediated gene targeting thus confers molecular biologists with the ability to site-specifically and permanently alter not only plant and mammalian genomes but also many other organisms by stimulating HR via a targeted genomic DSB. Site-specific engineering of the plant and mammalian genome in cells so far has been hindered by the low frequency of HR. In ZFN-mediated gene targeting, this is circumvented by using designer ZFNs to cut at the desired chromosomal locus inside the cells. The DNA break is then patched up using the new investigator-provided genetic information and the cells' own repair machinery. The accuracy and high efficiency of the HR process combined with the ability to design ZFNs that target most DNA sequences (if not all) makes ZFN technology not only a powerful research tool for site-specific manipulation of the plant and mammalian genomes, but also potentially for human therapeutics in the future, in particular for targeted engineering of the human genome of clinically transplantable stem cells.

Long-range identification of hepatocyte nuclear factor-3 (FoxA) high and low-affinity binding sites with a chimeric nuclease.

Identifying the complete set of transcription factors that bind the promoter and other regulatory regions of a gene of interest is an essential step in functional genomics. We have developed an original assay for the systematic detection of hepatocyte nuclear factor-3 (HNF-3) binding sites within cloned promoters. This assay is based on expression of a recombinant enzyme, HNF-3beta/FN, that is comprised of the rat HNF-3beta DNA-binding domain and the non-specific nuclease domain of the FokI restriction enzyme. Southern analysis of target plasmids with proven HNF-3 binding sites showed that HNF-3beta/FN was able to specifically cut both DNA strands in the vicinity of these binding sites, whereas mutagenized binding sites were no longer cleaved. Likewise, as yet undescribed HNF-3 binding sites were detected easily over a distance spanning several thousand bases. The functionality of such binding sites was confirmed by electromobility shift assay. Furthermore, the extent of cleavage by HNF-3beta/FN at a given binding site was tightly correlated with the affinity of a natural HNF-3beta molecule for this site. This novel approach can be extended to other transcription factors for long-range identification of functional transcription factor binding sites in genes.

Stimulation of homologous recombination through targeted cleavage by chimeric nucleases.

Chimeric nucleases that are hybrids between a nonspecific DNA cleavage domain and a zinc finger DNA recognition domain were tested for their ability to find and cleave their target sites in living cells. Both engineered DNA substrates and the nucleases were injected into Xenopus laevis oocyte nuclei, in which DNA cleavage and subsequent homologous recombination were observed. Specific cleavage required two inverted copies of the zinc finger recognition site in close proximity, reflecting the need for dimerization of the cleavage domain. Cleaved DNA molecules were activated for homologous recombination; in optimum conditions, essentially 100% of the substrate recombined, even though the DNA was assembled into chromatin. The original nuclease has an 18-amino-acid linker between the zinc finger and cleavage domains, and this enzyme cleaved in oocytes at paired sites separated by spacers in the range of 6 to 18 bp, with a rather sharp optimum at 8 bp. By shortening the linker, we found that the range of effective site separations could be narrowed significantly. With no intentional linker between the binding and cleavage domains, only binding sites exactly 6 bp apart supported efficient cleavage in oocytes. We also showed that two chimeric enzymes with different binding specificities could collaborate to stimulate recombination when their individual sites were appropriately placed. Because the recognition specificity of zinc fingers can be altered experimentally, this approach holds great promise for inducing targeted recombination in a variety of organisms.

Requirements for double-strand cleavage by chimeric restriction enzymes with zinc finger DNA-recognition domains.

This study concerns chimeric restriction enzymes that are hybrids between a zinc finger DNA-binding domain and the non-specific DNA-cleavage domain from the natural restriction enzyme FOK:I. Because of the flexibility of DNA recognition by zinc fingers, these enzymes are potential tools for cleaving DNA at arbitrarily selected sequences. Efficient double-strand cleavage by the chimeric nucleases requires two binding sites in close proximity. When cuts were mapped on the DNA strands, it was found that they occur in pairs separated by approximately 4 bp with a 5' overhang, as for native FOK:I. Furthermore, amino acid changes in the dimer interface of the cleavage domain abolished activity. These results reflect a requirement for dimerization of the cleavage domain. The dependence of cleavage efficiency on the distance between two inverted binding sites was determined and both upper and lower limits were defined. Two different zinc finger combinations binding to non-identical sites also supported specific cleavage. Molecular modeling was employed to gain insight into the precise location of the cut sites. These results define requirements for effective targets of chimeric nucleases and will guide the design of novel specificities for directed DNA cleavage in vitro and in vivo.

Phage display of ScFv peptides recognizing the thymidine(6-4)thymidine photoproduct.

Solar ultraviolet (UV) radiation induces DNA photoproducts in skin cells and is the predominant cause of human skin cancers. To understand human susceptibility to skin cancer and to facilitate the development of prevention measures, highly specific reagents to detect and quantitate UV-induced DNA adducts in human skin will be needed. One approach towards this end is the use of monoclonal antibody-based molecular dosimetry methods. To facilitate the development of photoproduct-specific antibody reagents we have: (i) cloned and sequenced a single chain variable fragment (ScFv) gene coding for one such high affinity monoclonal antibody, [alpha]UVssDNA-1 (mAb C3B6), recognizing the thymidine(6-4)thymidine photoproduct; (ii) expressed and displayed the cloned ScFv gene on the surface of phage; (iii) selected functional recombinant phage by panning; (iv) purified the ScFv peptide; (v) shown that the purified ScFv peptide binds to UV-irradiated polythymidylic acid but not unirradiated polythymidylic acid. This is the first demonstration of the use of phage display to select a ScFv recognizing DNA damage. In addition, this is the initial step towards immortalizing the antibody gene for genetic manipulation, structure-function studies and application to human investigations.

Chimeric restriction enzymes: what is next?

Chimeric restriction enzymes are a novel class of engineered nucleases in which the non-specific DNA cleavage domain of Fokl (a type IIS restriction endonuclease) is fused to other DNA-binding motifs. The latter include the three common eukaryotic DNA-binding motifs, namely the helix-turn-helix motif, the zinc finger motif and the basic helix-loop-helix protein containing a leucine zipper motif. Such chimeric nucleases have been shown to make specific cuts in vitro very close to the expected recognition sequences. The most important chimeric nucleases are those based on zinc finger DNA-binding proteins because of their modular structure. Recently, one such chimeric nuclease, Zif-QQR-F(N) was shown to find and cleave its target in vivo. This was tested by microinjection of DNA substrates and the enzyme into frog oocytes (Carroll et al., 1999). The injected enzyme made site-specific double-strand breaks in the targets even after assembly of the DNA into chromatin. In addition, this cleavage activated the target molecules for efficient homologous recombination. Since the recognition specificity of zinc fingers can be manipulated experimentally, chimeric nucleases could be engineered so as to target a specific site within a genome. The availability of such engineered chimeric restriction enzymes should make it feasible to do genome engineering, also commonly referred to as gene therapy.

A detailed study of the substrate specificity of a chimeric restriction enzyme.

Recently, the crystal structure of the designed zinc finger protein, DeltaQNK, bound to a preferred DNA sequence was reported. We have converted DeltaQNK into a novel site-specific endonuclease by linking it to the Fok I cleavage domain (FN). The substrate specificity and DNA cleavage properties of the resulting chimeric restriction enzyme (DeltaQNK-FN) were investigated, and the binding affinities of DeltaQNK and DeltaQNK-FN for various DNA substrates were determined. Substrates that are bound by DeltaQNK with high affinity are the same as those that are cleaved efficiently by DeltaQNK-FN. Substrates bound by DeltaQNK with lower affinity are cleaved with very low efficiency or not at all by DeltaQNK-FN. The binding of DeltaQNK-FN to each substrate was approximately 2-fold weaker than that for DeltaQNK. Thus, the fusion of the Fok I cleavage domain to the zinc finger motif does not change the DNA sequence specificity of the zinc finger protein and does not change its binding affinity significantly.

Chimeric restriction enzyme: Gal4 fusion to FokI cleavage domain.

Gal4, a yeast protein, activates transcription of genes required for metabolism of galactose and melibiose. It binds as a dimer to a consensus palindromic 17-base pair DNA sequence. It is a member of the third family of proteins that contain zinc-mediated peptide loops that interact specifically with nucleic acids. Gal4 has a very distinctive zinc coordination profile and mode of DNA-binding. Here, we report the creation of a novel site-specific endonuclease by linking the N-terminal 147 amino acids of Gal4 to the cleavage domain of FokI endonuclease. The fusion protein is active and under optimal conditions, binds to a 17 bp consensus DNA site and cleaves near this site. As expected, the cleavage occurs on either side of the consensus binding site(s).

Site-specific cleavage of DNA-RNA hybrids by zinc finger/FokI cleavage domain fusions.

Zinc-finger proteins of the Cys2His2 type bind DNA-RNA hybrids with affinities comparable to those for DNA duplexes. Such zinc-finger proteins were converted into site-specific cleaving enzymes by fusing them to the FokI cleavage domain. The fusion proteins are active and under optimal conditions cleave DNA duplexes in a sequence-specific manner. These fusions also exhibit site-specific cleavage of the DNA strand within DNA-RNA hybrids albeit at a lower efficiency (approximately 50-fold) compared to the cleavage of the DNA duplexes. These engineered endonucleases represent the first of their kind in terms of their DNA-RNA cleavage properties, and they may have important biological applications.

Hybrid restriction enzymes: zinc finger fusions to Fok I cleavage domain.

A long-term goal in the field of restriction-modification enzymes has been to generate restriction endonucleases with novel sequence specificities by mutating or engineering existing enzymes. This will avoid the increasingly arduous task of extensive screening of bacteria and other microorganisms for new enzymes. Here, we report the deliberate creation of novel site-specific endonucleases by linking two different zinc finger proteins to the cleavage domain of Fok I endonuclease. Both fusion proteins are active and under optimal conditions cleave DNA in a sequence-specific manner. Thus, the modular structure of Fok I endonuclease and the zinc finger motifs makes it possible to create "artificial" nucleases that will cut DNA near a predetermined site. This opens the way to generate many new enzymes with tailor-made sequence specificities desirable for various applications.

Insertion and deletion mutants of FokI restriction endonuclease.

FokI restriction endonuclease recognizes the nonpalindromic pentadeoxyribonucleotide, 5'-GGATG-3':5'-CATCC-3' in duplex DNA and cleaves 9 and 13 nucleotides away from the recognition site. We have reported the presence of two distinct and separable protein domains within this enzyme: one for the sequence-specific recognition of DNA (the DNA binding domain) and the other for the endonucleases activity (the cleavage domain). Our studies have suggested that the two domains are connected by a linker region, which appears to be amenable for repositioning of the DNA-sequence recognition domain with respect to the catalytic domain. Here, we report the construction of several insertion (4-, 8-, 12-, 18-, 19-, or 23-amino acid residues) and deletion (4- or 7-amino acid residues) mutants of the linker region of FokI endonuclease. The mutant enzymes were purified, and their cleavage properties were characterized. The mutants have the same DNA sequence specificity as the wild-type enzyme. However, compared with the wild-type enzyme, the insertion mutants cleave predominantly one nucleotide further away from the recognition site on both strands of the DNA substrate. The four-codon deletion mutant shows relaxed specificity at the cut site while the seven-codon deletion appears to inactivate the enzyme. The DNA binding and cleavage domains of FokI appear to be linked by a relatively malleable linker. No simple linear relationship exists between the linker length and the distance of the cut site from the recognition site. Furthermore, the four-codon insertion mutants cleave DNA substrates containing hemi-methylated FokI sites; they do not cleave fully methylated substrates. These results are best explained as a consequence of protein-protein interactions between the domains.

Chimeric restriction endonuclease.

Fok I restriction endonuclease recognizes the nonpalindromic pentadeoxyribonucleotide 5'-GGATG-3'.5'-CATCC-3' in duplex DNA and cleaves 9 and 13 nt away from the recognition site. Recently, we reported the presence of two distinct and separable domains within this enzyme: one for the sequence-specific recognition of DNA (the DNA-binding domain) and the other for the endonuclease activity (the cleavage domain). Here, we report the construction of a chimeric restriction endonuclease by linking the Drosophila Ultrabithorax homeodomain to the cleavage domain (FN) of Fok I restriction endonuclease. The hybrid enzyme, Ubx-FN, was purified, and its cleavage properties were characterized. The hybrid enzyme shows the same DNA sequence-binding preference as that of Ubx; as expected, it cleaves the DNA away from the recognition site. On the 5'-TTAATGGTT-3' strand the hybrid enzyme cleaves 3 nt away from the recognition site, whereas it cuts the complementary 5'-AACCATTAA-3' strand 8, 9, or 10 nt away from the binding site. Similarly engineered hybrid enzymes could be valuable tools in physical mapping and sequencing of large eukaryotic genomes.

New vectors for direct cloning of PCR products.

We describe the construction of two new vectors for direct cloning of polymerase chain reaction (PCR) products. This was done by inserting a synthetic DNA fragment containing two adjacent XcmI sites between the Asp718 and BamHI sites of the M13mp18 and M13mp19 phages. Cleavage of these M13 derivatives with XcmI will result in a linearized vector with a single thymidine nucleotide at the 3' ends. Thus, these vectors would be very useful for direct cloning of PCR-generated products with high efficiency.

C-terminal deletion mutants of the FokI restriction endonuclease.

We have constructed two C-terminal deletion mutants of the FokI restriction endonuclease by using the polymerase-chain-reaction technique and expressed them in Escherichia coli. The two mutant proteins (MP) of 41 and 30 kDa, were purified to homogeneity and their DNA-binding properties were characterized. The 41-kDa MP specifically binds the DNA sequence, 5'-GGATG/3'-CCTAC, like the wild-type (wt) FokI, but does not cleave DNA. The 30-kDa MP does not bind DNA. The affinity of the 41-kDa MP for the DNA substrate is comparable to that of wt FokI. The 41-kDa MP interacts with its substrate like the wt FokI, as revealed by hydroxyl radical footprinting experiments. In the presence of a DNA substrate, the 41-kDa MP is cleaved by trypsin into a 30-kDa N-terminal fragment and an 11-kDa C-terminal fragment. Addition of the HPLC-purified 11-kDa C-terminal fragment to the 30-kDa MP restores its sequence-specific DNA-binding property. These results confirm that the N-terminal 41-kDa fragment of the FokI ENase constitutes the DNA recognition domain of the ENase.

Alteration of the cleavage distance of Fok I restriction endonuclease by insertion mutagenesis.

Fok I restriction endonuclease recognizes the nonpalindromic pentadeoxyribonucleotide 5'-GGATG-3'.5'-CATCC-3' in duplex DNA and cleaves 9 and 13 nucleotides away from the recognition site. Recently, we reported the presence of two distinct and separable protein domains within this enzyme--one for the sequence-specific recognition and the other for endonuclease activity. Here, we report the construction of two insertion mutants of Fok I endonuclease. The mutant enzymes were purified, and their cleavage properties were characterized. The mutants have the same DNA sequence specificity as the wild-type enzyme. However, compared with the wild-type enzyme, they cleave one nucleotide further away from the recognition site on both strands of the DNA substrates. Thus, it is possible to alter the cleavage distance of Fok I by protein engineering.

Cloning, sequencing, overproduction, and purification of M. CviBI (GANTC) methyltransferase from Chlorella virus NC-1A [corrected].

We have cloned and sequenced the cvibIM gene from Chlorella virus NC-1A by selecting for the modification phenotype. The modification gene was cloned on a 7-kb BamHI fragment inserted into the BamHI site of the pUC13 plasmid. The cvibIM gene was localized at the 3' end of this fragment. Sequencing of this region revealed a large open reading frame that codes for methyltransferase (MTase; symbol M.) (predicting 260 amino acids). M.CviBI (GANTC) aa sequence is homologous to M.Dam(GATC), M.DpnII(GATC), and M.T4 (GATC), and not so to M.HinfI(GANTC), M.HhaII (GANTC), and M.DpnA(GATC). We also describe the use of the polymerase chain reaction technique to alter transcriptional and translational signals surrounding this gene so as to achieve overexpression in Escherichia coli. This construct yields M.CviBI at 2-3% of the total cellular protein. The MTase was purified by phosphocellulose, DEAE, and gel filtration chromatography. Its size by SDS-PAGE is approx. 28 kDa, in good agreement with that predicted from the nucleotide sequence.

Functional domains in Fok I restriction endonuclease.

The PCR was used to alter transcriptional and translational signals surrounding the Flavobacterium okeanokoites restriction endonuclease (fokIR) gene, so as to achieve high expression in Escherichia coli. By changing the ribosome-binding site sequence preceding the fokIR gene to match the consensus E. coli signal and by placing a positive retroregulator stem-loop sequence downstream of the gene, Fok I yield was increased to 5-8% of total cellular protein. Fok I was purified to homogeneity with phosphocellulose, DEAE-Sephadex, and gel chromatography, yielding 50 mg of pure Fok I endonuclease per liter of culture medium. The recognition and cleavage domains of Fok I were analyzed by trypsin digestion. Fok I in the absence of a DNA substrate cleaves into a 58-kDa carboxyl-terminal and 8-kDa amino-terminal fragment. The 58-kDa fragment does not bind the DNA substrate. Fok I in the presence of a DNA substrate cleaves into a 41-kDa amino-terminal fragment and a 25-kDa carboxyl-terminal fragment. On further digestion, the 41-kDa fragment degrades into 30-kDa amino-terminal and 11-kDa carboxyl-terminal fragments. The cleaved fragments both bind DNA substrates, as does the 41-kDa fragment. Gel-mobility-shift assays indicate that all the protein contacts necessary for the sequence-specific recognition of DNA substrates are encoded within the 41-kDa fragment. Thus, the 41-kDa amino-terminal fragment constitutes the Fok I recognition domain. The 25-kDa fragment, purified by using a DEAE-Sephadex column, cleaves nonspecifically both methylated (pACYCfokIM) and nonmethylated (pTZ19R) DNA substrates in the presence of MgCl2. Thus, the 25-kDa carboxyl-terminal fragment constitutes the Fok I cleavage domain.

Overproduction, purification and characterization of M.HinfI methyltransferase and its deletion mutant.

We have used the polymerase chain reaction to alter transcriptional and translational signals surrounding the hinfIM gene [encoding M.HinfI methyltransferase (MTase)] so as to achieve overexpression in Escherichia coli. The PCR-generated hinfIM gene was subcloned in a high-expression vector under control of the hybrid trp-lac promoter. In addition, the positive retroregulator stem-loop sequence derived from the crystal protein-encoding gene of Bacillus thuringiensis was inserted downstream from hinfIM. Using a similar approach, we have also constructed overproducer clones of a deletion mutant of M.HinfI MTase that has 97 amino acids from the C terminus deleted. The plasmid from the mutant clones is fully protected from HinfI restriction endonuclease digestion. It appears that the functional properties (the recognition and catalytic functions) are encoded within this mutant gene. The overproducer clones yield the wild type (wt) and the mutant enzymes to about 10% of total cellular protein upon induction with 1 mM IPTG. The wt M.HinfI and the mutant MTase were purified to near electrophoretic homogeneity by phosphocellulose, DEAE and gel chromatography. Their monomer sizes by SDS/polyacrylamide-gel electrophoresis are 43 kDa and 31 kDa, respectively, in good agreement with that predicted from the nucleotide sequence. DNA methylation experiments with purified enzymes using single-strand and double-strand M13mp18 DNA substrates indicate that while wt enzyme methylates both forms of DNA substrates, the mutant enzyme appears to preferentially methylate ss DNA substrate.

Construction of an efficient overproducer clone of HinfI restriction endonuclease using the polymerase chain reaction.

We describe the use of the polymerase chain reaction (PCR) technique to alter transcriptional and translational signals surrounding a gene so as to achieve overexpression in Escherichia coli. By changing the ribosome-binding site sequence preceding the hinfIR gene to match the consensus E. coli signal and by adding a transcription terminator sequence immediately following the gene, the yield of HinfI was increased about tenfold over that obtained from the natural Haemophilus influenzae signals. The addition of the positive retroregulator stem-loop sequence derived from the crystal protein-encoding gene of Bacillus thuringiensis downstream from the hinfIR gene further increased yields by twofold to a level of 13% of the total cellular protein.