Molecular cloning of PCR products.

It is often desirable to clone PCR products to establish a permanent source of cloned DNA for hybridization studies, to obtain high-quality DNA sequencing results, or to separate products when PCR amplification yields a complex mixture. The efficiency of direct cloning of PCR products can be improved by generating suitable ends on the amplified fragments. This unit describes the strategies for generating and manipulating suitable ends on the PCR fragments.

Effective amplification of 20-kb DNA by reverse transcription PCR.

Polymerase chain reaction has been applied to the amplification of long DNA fragments from a variety of sources, including genomic, mitochondrial, and viral DNAs. However, polymerase chain reaction amplification from cDNA templates produced by reverse transcription has generally been restricted to products of less than 10 kilobases. In this paper, we report a system to effectively amplify fragments up to 20 kilobases from human coronavirus 229E genomic RNA. We demonstrate that the integrity of the RNA template and the prevention of false priming events during reverse transcription are the critical parameters to achieve the synthesis of long cDNAs. The optimization of the polymerase chain reaction conditions enabled us to improve the specificity and yield of product but they were not definitive. Finally, we have shown that the same reverse transcription polymerase chain reaction technology can be used for the amplification of extended regions of the dystrophin mRNA, a cellular RNA of relatively low abundance.

Archaebacterial DNA polymerases tightly bind uracil-containing DNA.

We show that archaebacterial DNA polymerases are strongly inhibited by the presence of small amounts of uracil-containing DNA. Inhibition appears to be competitive, with the DNA polymerase exhibiting approximately 6500-fold greater affinity for binding the inhibitor than a DNase I-activated DNA substrate. All six archaebacterial DNA polymerases tested were inhibited, while no eubacterial, eukaryotic, or bacteriophage enzymes showed this effect. Only a small inhibition resulted when uracil was present as the deoxynucleoside triphosphate, dUTP. The rate of DNA synthesis was reduced by approximately 40% when dUTP was used in place of dTTP for archaebacterial DNA polymerases. Furthermore, an incorporated dUMP served as a productive 3'-primer terminus for subsequent elongation. In contrast, the presence of an oligonucleotide containing as little as a single dUrd residue was extremely inhibitory to DNA polymerase activity on other primer-template DNA.

Novel methods for cloning and engineering genes using the polymerase chain reaction.

Use of the polymerase chain reaction (PCR) has become increasingly widespread in virtually all aspects of molecular biology. Recently, novel ligation-independent methods have been developed for the cloning of DNA fragments amplified using PCR. Ligation-independent cloning utilizing the enzyme uracil DNA glycosylase (termed UDG cloning) provides an efficient method for gene cloning and recombinant PCR. This technology is now being applied to site-directed mutagenesis, the generation of nested deletions, and the engineering of novel gene constructs. The ease and flexibility of this methodology, combined with PCR amplification, simplify gene cloning and engineering techniques.

Assembly and cloning of coding sequences for neurotrophic factors directly from genomic DNA using polymerase chain reaction and uracil DNA glycosylase.

The polymerase chain reaction (PCR) was used to amplify individual exons of the gene (CNTF) coding for human ciliary neurotrophic factor (CNTF) directly from genomic DNA. Inclusion of deoxyuracil in place of thymine in the PCR primers permits removal of dU residues in the primer after amplification using uracil DNA glycosylase, generating single-stranded 3' overhangs. Thus, the individual exons were assembled to generate the full-length CNTF sequence. A similar strategy was also used to generate a chimeric gene (BDNF) encoding brain-derived neurotrophic factor (BDNF) with the pre-pro sequence of nerve growth factor (NGF). The method described allows direct amplification of coding sequence from genomic DNA and ordered assembly of amplified exons to generate a clone containing the complete coding sequence of the gene without the need for splicing; a clone which is equivalent to a cDNA clone.

Nucleotide imbalance and polymerase chain reaction: effects on DNA amplification and synthesis of high specific activity radiolabeled DNA probes.

Synthesis of radiolabeled DNA probes via polymerase chain reaction (PCR) is a convenient alternative to the more conventional methods of random primer-labeling and nick translation. PCR requires less template and allows the synthesis of radiolabeled probes from specific sequences contained within cloning vectors and genomic DNA. Under nucleotide imbalance conditions where the concentration of the radiolabeled nucleotide was 0.825 microM and the other dNTPs were each > 25 microM, amplification by Taq DNA polymerase was inhibited. Reducing the concentrations of the unlabeled dNTPs resulted in greater yields of amplification product with maximal yield obtained when the concentration of three unlabeled nucleotides was two- to eightfold higher than that of the limiting labeled nucleotide. When we utilized this amplification method for synthesis of an 800-bp glyceraldehyde-3-phosphate (GAPDH) dehydrogenase probe, 87% of the added [32P]dCTP was incorporated into amplification product. Application of this method for synthesis of high specific activity probes ( > 4 x 10(9) cpm/micrograms) up to 2.6 kb in length is demonstrated and utility of the 800-bp GAPDH probe for hybridization to Northern blots for detection of GAPDH mRNA is presented.

Selective RNA amplification: a novel method using dUMP-containing primers and uracil DNA glycosylase.

The application of PCR to a wide variety of biological problems and molecular techniques has gained wide acceptance. RNA-PCR, a technique in which first-strand cDNA synthesis is followed by PCR amplification, has enabled detection and characterization of rare transcripts. One problem confronting the researcher involves specific amplification of transcribed sequences in the presence of small amounts of genomic DNA of identical sequence. We describe a novel technique, selective RNA amplification, which will specifically amplify RNA sequences in a background of homologous DNA. The method involves first-strand cDNA synthesis from a specific dUMP-containing oligonucleotide that contains unique user-defined 5' sequence (adapter sequence) not found in the message of interest. RNA template is degraded using RNase H, which is specific for RNA/DNA hybrids. This is followed by second-strand synthesis using a gene-specific primer (GSP). The original adapter primer is digested with uracil DNA glycosylase (UDG) to prevent its participation in subsequent amplification. PCR is then performed using the GSP and a second primer corresponding to the unique adapter sequence. In this paper, we apply this method to the amplification of RNA derived from human papilloma virus sequences. Using Southern analysis, we demonstrate specific amplification of 10(5) molecules of an in vitro-transcribed RNA. Denatured DNA of identical sequence and concentration was not amplified using the RNA-specific method. The method could eliminate the need for stringent purification of RNA and enables amplification of rare messages from RNA preparations containing homologous DNA of identical sequence and size.

A new PCR based method for the generation of nested deletions.

We have developed a simple, PCR-based protocol, random primed/anchored-PCR (RPA-PCR), that allows the selective amplification and efficient cloning of segments that are adjacent to any known sequence. We demonstrate that RPA-PCR can be used to prepare a nested set of evenly spaced deletions suitable for DNA sequencing. However, it should also be possible to use this technique for a number of other purposes: generating deletions for the analysis of eukaryotic promoters, extending cDNA clones in the 5' direction, cloning the insertion sites of retroviral proviruses and transposons, and analyzing intron/exon boundaries.

Use of random primer extension for concurrent amplification and nonradioactive labeling of nucleic acids.

A method for efficient nonradioactive labeling of DNA with biotin using random primer extension has been developed. Under the conditions described, a significant amount of DNA synthesis occurs during incorporation of the nonradioactive label, resulting in amplification of the original template DNA. The effect of primer size, substrate concentration, enzyme concentration, and ratio of biotinylated nucleotide to normal nucleotide on the amount of DNA synthesis was determined. Amplifications of 10- to > 300-fold were attained, depending on the starting template concentration. Template may be varied from 1 to 500 ng per reaction. The size of the resulting biotinylated probes is 100-1000 nucleotides with a significant proportion in the 100-300 nucleotide range. The biotinylated probes were used to detect single-copy genes on Southern blot hybridizations and to identify specific loci in metaphase chromosome spreads by in situ hybridization followed by fluorescent detection with streptavidin-fluorescein isothiocyanate. Random primer amplification and labeling provides a convenient method for preparation of biotinylated probes from small amounts of template DNA.

A novel method for site-directed mutagenesis using PCR and uracil DNA glycosylase.

A novel method for site-directed mutagenesis of DNA sequences based on the use of the PCR is described. The method uses two oligonucleotide primers that contain the desired sequence change and overlap at their 5' ends. In addition, the thymine residues in the overlap region have been substituted with deoxyuracil. Amplification of the template plasmid by PCR results in incorporation of the primers and the desired mutation into the PCR product. Excision of the deoxyuracil residues in the PCR products by uracil DNA glycosylase (UDG) destablizes base-pairing at the ends of DNA molecules and thus generates 3' protruding ends in the opposite strand. Due to overlapping nature of the primers, the resulting 3' protruding ends are complementary and can anneal rapidly after treatment with UDG. When the entire plasmid is amplified, a linear mutant PCR product is generated that circularizes after treatment with UDG. Circularized molecules can then be transformed into competent cells without ligation, generating transformants with the mutant genotype. Alternatively, the gene of interest is amplified in two segments using overlapping mutant primers and cloned in the desired orientation into pAMP1 by UDG cloning. Application of this method to site-specific mutagenesis of the lacZ alpha gene and the human c-raf oncogene was demonstrated. The accuracy of the mutations was confirmed by nucleotide sequence analysis as well as phenotypic assays. The method is rapid, highly efficient (> 99%), and applicable to genes cloned in any vector as well as to genomic DNA or RNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Uracil DNA glycosylase-mediated cloning of polymerase chain reaction-amplified DNA: application to genomic and cDNA cloning.

A simple and rapid method for cloning of amplification products directly from the polymerase chain reaction (PCR) has been developed. The method is based on the addition of a 12-base dUMP-containing sequence (CUACUACUACUA) to the 5' end of PCR primers. Incorporation of these primers during PCR results in the selective placement of dUMP residues into the 5' end of amplification products. Selective degradation of the dUMP residues in the PCR products with uracil DNA glycosylase (UDG) disrupts base pairing at the termini and generates 3' overhangs. Annealing of 3' protruding termini to vector DNA containing complementary 3' ends results in chimeric molecules which can be transformed, with high efficiency, without in vitro ligation. Directional cloning of PCR products has also been accomplished by incorporating different dU-containing sequences at the end of each PCR primer. Substitution of all dT residues in PCR primers with dU eliminates cloning of aberrant "primer dimer" products and enriches cloning of genuine PCR products. The method has been applied to cloning of inter-Alu DNA sequences from human placental DNA. Using a single primer, DNA sequences between appropriately oriented Alu sequences were amplified and cloned. Cloning of cDNA for the glyceraldehyde-3'-phosphate dehydrogenase gene from rat brain RNA was also demonstrated. The 3' end region of this gene was amplified by the 3' RACE method and the amplified DNA was cloned after UDG digestion. Characterization of cloned DNAs by sequence analysis showed accurate repair of the cloning junctions. The ligase-free cloning method with UDG should prove to be a widely applicable procedure for rapid cloning of PCR-amplified DNA.

Rapid and efficient cloning of Alu-PCR products using uracil DNA glycosylase.

By incorporating dUMP residues into the 5' end of PCR primers, one can generate products which, after treatment with uracil DNA glycosylase (UDG), contain 3' overhangs. These overhangs can be annealed to vector molecules with complementary overhangs generated in a similar fashion and transformed directly into Escherichia coli without the need for ligase. We have tested this method of ligation-independent cloning by using UDG to create complementary single-stranded sticky ends between vector and Alu-PCR products generated from cosmid clones containing DNA from human chromosome 21. Using a single primer, Alu-PCR amplifies the sequence between appropriately oriented, repetitive (Alu) sequences in human DNA that are no more than 2 to 3 kb apart. Nineteen Alu-PCR products were observed in four human chromosome 21 cosmids. Thirteen of these products were detected among 48 subclones picked at random after cloning of the Alu-PCR products using UDG. The size or abundance of an Alu-PCR product did not appear to affect significantly the efficiency of cloning. Eight of the subclones were tested and all hybridized to human chromosome 21 DNA. UDG cloning should prove to be a general PCR cloning method that allows one to rapidly subclone small fragments from human genomic DNA.

Immunological capture of nucleic acid hybrids and application to nonradioactive DNA probe assay.

Antibodies specific for DNA:RNA hybrids were coated onto polystyrene test tubes and applied to hybridization assays involving DNA and RNA. Synthetic DNA probes complementary to 16S rRNA of Campylobacter were labeled with biotin and hybridized to ribosomal RNA directly in lysates of bacterial cells. After hybridization, DNA:RNA hybrids were captured with immobilized anti-DNA:RNA antibody, and the biotinylated probe was detected with streptavidin-horseradish peroxidase (EC 1.11.1.7) conjugate. The assay was optimized to detect as few as 70,000 Campylobacter cells in a sample. We compared the utility of this hybridization assay with that of conventional microbiology methods by examination of 1448 stool samples from hospital clinical laboratories. The DNA hybridization assay had a sensitivity of 98.7% (75/76) and a specificity of 98.2% (1347/1372) and overall agreed with 98.2% of the conventional results for a test population that had a 5.2% incidence (76/1448) of Campylobacter infection. The assay is simple to perform and yields results within 2.5 h.

Immunochemical approaches to gene probe assays.

Antibodies specific for helical nucleic acids can be applied to assays for hybridized DNA and/or RNA. The assays can use either radioactive or nonradioactive detection systems. Antibodies specific for RNA-DNA hybrids are applicable to assays for measuring hybrid helices that are immobilized on plastic or nitrocellulose, whether the helices are preformed in solution or are formed on the solid-phase support. Alternatively, anti-RNA-DNA hybrid antibodies can be immobilized and used to capture hybrids formed in solution, resulting in an assay with a high signal-to-noise ratio. It has been applied to a test for the presence of ribosomal RNA of Campylobacter jejuni in biological samples.

Plasmid segregation into minicells is associated with membrane attachment and independent of plasmid replication.

The role of plasmid replication in the segregation of plasmids into Escherichia coli minicells was investigated with temperature-sensitive replication mutants derived from E. coli plasmids ColE1 and pSC101. For as long as six generations of growth, at permissive or nonpermissive temperatures (when greater than 80% of plasmid replication was inhibited), the same amount of previously 3H-labeled plasmid DNA segregated into minicells. Density gradient separations of wild-type and temperature-sensitive plasmid DNA from both replicons segregated into the minicells showed that about 20 to 25% was stably associated with the minicell membrane at both temperatures. Electron microscopy showed this DNA to consist of circular plasmid molecules attached to the minicell membrane. These combined findings suggest that segregation of plasmids into minicells and their association with the minicell membrane are interrelated and independent of plasmid replication.