Comparison of the 5' nuclease activities of taq DNA polymerase and its isolated nuclease domain.

Many eubacterial DNA polymerases are bifunctional molecules having both polymerization (P) and 5' nuclease (N) activities, which are contained in separable domains. We previously showed that the DNA polymerase I of Thermus aquaticus (TaqNP) endonucleolytically cleaves DNA substrates, releasing unpaired 5' arms of bifurcated duplexes. Here, we compare the substrate specificities of TaqNP and the isolated 5' nuclease domain of this enzyme, TaqN. Both enzymes are significantly activated by primer oligonucleotides that are hybridized to the 3' arm of the bifurcation; optimal stimulation requires overlap of the 3' terminal nucleotide of the primer with the terminal base pair of the duplex, but the terminal nucleotide need not hybridize to the complementary strand in the substrate. In the presence of Mn2+ ions, TaqN can cleave both RNA and circular DNA at structural bifurcations. Certain anti-TaqNP mAbs block cleavage by one or both enzymes, whereas others can stimulate cleavage of nonoptimal substrates.

Polymorphism identification and quantitative detection of genomic DNA by invasive cleavage of oligonucleotide probes.

Flap endonucleases (FENs) isolated from archaea are shown to recognize and cleave a structure formed when two overlapping oligonucleotides hybridize to a target DNA strand. The downstream oligonucleotide probe is cleaved, and the precise site of cleavage is dependent on the amount of overlap with the upstream oligonucleotide. We have demonstrated that use of thermostable archaeal FENs allows the reaction to be performed at temperatures that promote probe turnover without the need for temperature cycling. The resulting amplification of the cleavage signal enables the detection of specific DNA targets at sub-attomole levels within complex mixtures. Moreover, we provide evidence that this cleavage is sufficiently specific to enable discrimination of single-base differences and can differentiate homozygotes from heterozygotes in single-copy genes in genomic DNA.

Differentiation of bacterial 16S rRNA genes and intergenic regions and Mycobacterium tuberculosis katG genes by structure-specific endonuclease cleavage.

We describe here a new approach for analyzing nucleic acid sequences using a structure-specific endonuclease, Cleavase I. We have applied this technique to the detection and localization of mutations associated with isoniazid resistance in Mycobacterium tuberculosis and for differentiating bacterial genera, species and strains. The technique described here is based on the observation that single strands of DNAs can assume defined conformations, which can be detected and cleaved by structure-specific endonucleases such as Cleavase I. The patterns of fragments produced are characteristic of the sequences responsible for the structure, so that each DNA has its own structural fingerprint. Amplicons, containing either a single 5'-fluorescein or 5'-tetramethyl rhodamine label were generated from a 620-bp segment of the katG gene of isoniazid-resistant and -sensitive M. tuberculosis, the 5' 350 bp of the 16S rRNA genes of Escherichia coli O157:H7, Salmonella typhimurium, Salmonella enteritidis, Salmonella arizonae, Shigella sonnei, Shigella dysenteriae, Campylobacter jejuni, staphylococcus, hominis, Staphylococcus warneri, and Staphylococcus aureus and an approximately 550-bp DNA segment comprising the intergenic region between the 16S and 23S rRNA genes of Salmonella typhimurium, Salmonella enteritidis, Salmonella arizonae, Shigella sonnei, and Shigella dysenteriae serotypes 1, 2, and 8. Changes in the structural fingerprints of DNA fragments derived from the katG genes of isoniazid-resistant M. tuberculosis isolates were clearly identified and could be mapped to the site of the actual mutation relative to the labeled end. Bland patterns which clearly differentiated bacteria to the level of genus and, in some cases, species were generated from the 16S genes. Cleavase I analysis of the intergenic regions of Salmonella and Shigella species differentiated genus, species, and serotypes. Structural fingerprinting by digestion with Cleavase I is a rapid, simple, and sensitive method for analyzing nucleic acid sequences and may find wide utility in microbial analysis.

Structure-specific endonucleolytic cleavage of nucleic acids by eubacterial DNA polymerases.

Previously known 5' exonucleases of several eubacterial DNA polymerases have now been shown to be structure-specific endonucleases that cleave single-stranded DNA or RNA at the bifurcated end of a base-paired duplex. Cleavage was not coupled to synthesis, although primers accelerated the rate of cleavage considerably. The enzyme appeared to gain access to the cleavage site by moving from the free end of a 5' extension to the bifurcation of the duplex, where cleavage took place. Single-stranded 5' arms up to 200 nucleotides long were cleaved from such a duplex. Essentially any linear single-stranded nucleic acid can be targeted for specific cleavage by the 5' nuclease of DNA polymerase through hybridization with an oligonucleotide that converts the desired cleavage site into a substrate.

Use of in vitro protein synthesis from polymerase chain reaction-generated templates to study interaction of Escherichia coli transcription factors with core RNA polymerase and for epitope mapping of monoclonal antibodies.

The interaction of two Escherichia coli transcription factors (sigma 54 and sigma 32) with the core RNA polymerase was studied here. We examined the core binding ability of various fragments of these two transcription factors using a novel method for the in vitro synthesis of truncated proteins in a coupled transcription-translation (S-30) system. The method uses DNA templates generated by the polymerase chain reaction to direct synthesis of precisely truncated fragments of the encoded protein. Primers for the polymerase chain reaction contain transcription and translation signals so that the resulting product can be incubated in the S-30 directly. The synthesized proteins were used for mapping of both functional domains and epitopes of monoclonal antibodies to sigma 54 and sigma 32.

Isolation and characterisation of the crnA-niiA-niaD gene cluster for nitrate assimilation in Aspergillus nidulans.

Genomic clones containing the entire crnA-niiA-niaD gene cluster of Aspergillus nidulans have been isolated, and the structures of the niiA and niaD genes have been determined by nucleotide sequence analysis. This gene cluster is required for the assimilation of nitrate in A. nidulans, and the three genes encode a product required for nitrate uptake and the enzymes, nitrite reductase and nitrate reductase, respectively. The putative coding sequences, as deduced by comparison to cDNA clones of both niiA and niaD, are interrupted by multiple small introns, and the two genes are divergently transcribed. Identification and characterization of specific mRNAs involved in nitrate assimilation indicates that only monocistronic transcripts are involved, and that the approximate sizes of these transcripts are 1.6 kb, 3.4 kb and 2.8 kb for crnA, niiA and niaD, respectively. The results also indicate that control of niiA and niaD gene expression is mediated by the levels of mRNA accumulation, in response to the source of nitrogen in the growth medium. Two types of transcripts for niiA were observed.

Direct cDNA cloning of the rearranged immunoglobulin variable region.

A major problem in the study of multigene families is the effort required to clone and sequence these genes. We describe a method to rapidly clone and sequence immunoglobulin variable region gene sequences without constructing cDNA libraries. Because immunoglobulin variable-region genes are flanked by conserved sequences, we have been able to apply the polymerase chain reaction (PCR) to clone and sequence both the light- and heavy-chain rearranged immunoglobulin genes from small numbers of hybridoma cells. This method will greatly facilitate the construction of chimeric mouse/human monoclonal antibodies for immunoglobulin structural studies as well as for therapeutic use.

DNA sequencing with Thermus aquaticus DNA polymerase and direct sequencing of polymerase chain reaction-amplified DNA.

The highly thermostable DNA polymerase from Thermus aquaticus (Taq) is ideal for both manual and automated DNA sequencing because it is fast, highly processive, has little or no 3'-exonuclease activity, and is active over a broad range of temperatures. Sequencing protocols are presented that produce readable extension products greater than 1000 bases having uniform band intensities. A combination of high reaction temperatures and the base analog 7-deaza-2'-deoxyguanosine was used to sequence through G + C-rich DNA and to resolve gel compressions. We modified the polymerase chain reaction (PCR) conditions for direct DNA sequencing of asymmetric PCR products without intermediate purification by using Taq DNA polymerase. The coupling of template preparation by asymmetric PCR and direct sequencing should facilitate automation for large-scale sequencing projects.

p1B15: a cDNA clone of the rat mRNA encoding cyclophilin.

We present the complete nucleotide sequence of a cDNA encoding rat cyclophilin. The 743-nucleotide sequence contains a 42-nucleotide 5' noncoding region, a 492 nucleotide open reading frame corresponding to a translation product of 164 amino acids with a molecular weight of 17,874, and a 3' noncoding region of 209 nucleotides. Primer extension studies reveal the presence of one minor and two major transcription start sites. Southern blot analyses are consistent with as many as 20 copies of the cyclophilin gene and possible pseudogenes. Cyclophilin mRNA is expressed in virtually all types of tissues of rat and monkey and appears to have been highly conserved during mammalian evolution.

Two forms of 1B236/myelin-associated glycoprotein, a cell adhesion molecule for postnatal neural development, are produced by alternative splicing.

The structures of two rat brain-specific 1B236 mRNAs, alternative splice products from a single gene regulated differently during postnatal brain development, were deduced from full-length cDNA clones. The 626- and 582-amino acid-long encoded proteins are indistinguishable from two forms of myelin-associated glycoprotein, a cell adhesion molecule involved in axonal-glial and glial-glial interactions in postnatal brain development, particularly in myelination. The two proteins share a single membrane-spanning domain and a glycosylated N terminus but differ in the structures of their C termini. The N terminus consists of five domains related in sequence to each other and to immunoglobulin-like molecules, especially the neural cell adhesion molecule N-CAM, suggesting a common structure for cell adhesion molecules.

Expression of small cytoplasmic transcripts of the rat identifier element in vivo and in cultured cells.

We examined the level of expression of small RNA transcripts hybridizing to a rodent repetitive DNA element, the identifier (ID) sequence, in a variety of cell types in vivo and in cultured mammalian cells. A 160-nucleotide (160n) cytoplasmic poly(A)+ RNA (BC1) appeared in late embryonic and early postnatal rat brain development, was enriched in the cerebral cortex, and appeared to be restricted to neural tissue and the anterior pituitary gland. A 110n RNA (BC2) was specifically enriched in brain, especially the postnatal cortex, but was detectable at low levels in peripheral tissues. A third, related 75n poly(A)- RNA (T3) was found in rat brain and at lower levels in peripheral tissues but was very abundant in the testes. The BC RNAs were found in a variety of rat cell lines, and their level of expression was dependent upon cell culture conditions. A rat ID probe detected BC-like RNAs in mouse brain but not liver and detected a 200n RNA in monkey brain but not liver at lower hybridization stringencies. These RNAs were expressed by mouse and primate cell lines. Thus, tissue-specific expression of small ID-sequence-related transcripts is conserved among mammals, but the tight regulation found in vivo is lost by cells in culture.

The tetracycline repressor of pSC101.

We have determined the nucleotide sequence of the gene for the repressor of the pSC101 tetracycline resistance element (tetR). The repressor gene is transcribed divergently from the gene that encodes the resistance protein and encodes a putative protein of 219 amino acids. The genetic organizations of the three major types of bacterial tetracycline resistance elements thus appear to be equivalent, even though they do not show substantial nucleic acid similarity. The pSC101 repressor protein is 80% identical with the Tn 1721 repressor over its N-terminal 150 residues, whereas the C-termini of the two species are only 35% identical. Examination of the nucleic acid sequences of the regions between the two divergent promoters suggests a model in which two dimers of the tetracycline repressor molecule interact at two adjacent dyad repeats. The dimers may interact with each other, thus strengthening their grip on the operator, and affect transcription of the repressor gene. Comparison of the tetracycline (Tet) repressor with the lambda repressor suggests that the N-terminal region of the Tet repressor forms a helix-turn-helix structure and interacts with DNA in the major groove. The region of the Tet repressor implicated in DNA binding shows significant sequence similarity to a region of histone H4, suggesting that the histone may bind to DNA by means of a similar structural motif.

Nucleotide sequence of the transforming gene of m1 murine sarcoma virus.

The v-mosm1 nucleotide sequence codes for a protein that is 376 amino acids long. Although the N-terminus is homologous with that of the v-mos124 protein, the C-terminus is substantially different from the C-termini of all other examined mos proteins, suggesting that this region is nonessential and perhaps cleaved. Overall, v-mosm1 has greater homology with c-mos than does v-mos124, but mutually exclusive differences between c-mos and each of the v-mos genes preclude linear descent and suggest a common ancestral murine sarcoma virus.