[The Role of Cysteine Residues in the Interaction of Nicking Endonuclease BspD6I with DNA].

Nicking endonucleases (NEs) are a small, poorly studied family of restriction endonucleases. The enzymes recognize a target sequence in DNA, but catalyze the hydrolysis of only one strand. The mechanism of their action is important to study because NEs with new specificities are necessary to design to solve the practical tasks of biotechnology. One of the modern approaches for investigation of protein-nucleic acid interactions is fluorescence spectroscopy, which involves the introduction of fluorophores into proteins, mainly through Cys residues due to the high reactivity of their thiol group. To implement this approach, it is necessary to clarify the role of Cys residues in the functioning of the native protein and the possible consequences of their modification. Crosslinking was used to study whether Cys residues are close to DNA in the complex with NE BspD6I. Reactions were carried out using the wild-type enzyme, its mutant form NE BspD6I(C11S/C160S), and modified DNA duplexes containing the 2-pyridyldisulfide group at the C2' atom of the sugar-phosphate moiety in different positions of the oligonucleotide strand. The Cys residues of NE BspD6I were for the first time shown to be in close proximity to DNA during the binding process, including the step of a nonspecific complex formation. The substitutions C11S and C160S in the N-terminal domain of the enzyme slightly decreased the efficiency of substrate hydrolysis. Construction of a cysteine-free NE BspD6I variant and examination of its properties will provide additional information about the functional significance of the Cys residues for this unique enzyme.

Nicking endonucleases.

Nicking endonucleases are a new type of enzymes. Like restriction endonucleases, they recognize short specific DNA sequence and cleave DNA at a fixed position relatively to the recognition sequence. However, unlike restriction endonucleases, nicking endonucleases cleave only one predetermined DNA strand. Until recently, nicking endonucleases were suggested to be naturally mutated restriction endonucleases which had lost their ability to dimerize and as a result the ability to cleave the second strand. We have shown that nicking endonucleases are one of the subunits of heterodimeric restriction endonucleases. Mechanisms used by various restriction endonucleases for double-stranded cleavage, designing of artificial nicking endonucleases on the basis of restriction endonucleases, and application of nicking endonucleases in molecular biology are reviewed.

Plasmid pRARE as a vector for cloning to construct a superproducer of the site-specific nickase N.BspD6I.

The gene of methylase M.SccL1I that protects DNA against hydrolysis with the nickase N.BspD6I was inserted into plasmid pRARE carrying genes of tRNA, which are rare in E. coli. The insertion of the gene sscML1I into pRARE was reasoned by incompatibility of pRARE and the plasmid carrying the gene sscML1I, because both plasmids contained the same ori-site. Upon transformation of E. coli TOP10F cells with both the recombinant plasmid pRARE/MSsc and the expression vector pET28b containing the nickase gene bspD6IN under the phage T7 promoter, a strain of E. coli was obtained which produced 7 x 10(5) units of the nickase N.BspD6I per 1 g wet biomass, and this yield was two orders of magnitude higher than the yield of the enzyme from the strain free of pRARE/MSsc.

Cloning and sequencing of the gene of site-specific nickase N.BspD6I.

A fragment of chromosomal DNA from Bacillus species D6 containing the gene of nickase N.BspD6I and the regions adjacent to its 5;- and 3;-ends was cloned and sequenced. The nucleotide sequence of the nickase gene, except of one neutral change, is homologous to the nicking endonuclease N.BstNBI gene sequenced by Higgens et al. (2001). After integration of a PCR-copy of the nickase gene into an expression vector pET28b under the control of the phage T7 promoter, specific nicking activity was detected in the lysates of transformed E. coli cells.

Some properties of site-specific nickase BspD6I and the possibility of its use in hybridization analysis of DNA.

A new method for hybridization analysis of nucleic acids is proposed on the basis of the ability of site-specific nickases to cleave only one DNA strand. The method is based on the use of a labeled oligonucleotide with the recognition site of the nickase hybridized with the target (DNA or RNA) at an optimal temperature of the enzyme (55 degrees C). The two shorter oligonucleotides formed after the cleavage with the nickase do not complex with the target. Thus, a multiple cleavage of the labeled oligonucleotide takes place on one target molecule. The cleavage of the nucleotide is recorded either by polyacrylamide gel electrophoresis (when a radioactive labeled oligonucleotide is used) or by fluorescence measurements (if the oligonucleotide has the structure of a molecular beacon). The new method was tested on nickase BspD6I and a radioactive oligonucleotide complementary to the polylinker region of the viral DNA strand in bacteriophage M13mp19. Unfortunately, nickase BspD6I does not cleave DNA in the RNA-DNA duplexes and therefore cannot be used for detection of RNA targets.

Site-specific nickase from bacillus species strain d6.

Three site-specific endonucleases were found in thermophilic strain Bacillus species D6. One of them, BspD6II, is an isoschizomer of Eco57I. The second, BspD6III, is present in the strain in very small amount; therefore, it has not been characterized. This paper is devoted to the third, BspD6I, which recognizes pentanucleotide site 5'-GAGTC-3' and cleaves only one DNA strand at a distance of 4 nucleotides from the site in the 3'-direction in the chain with the GAGTC sequence, i.e., it behaves as a site-specific nickase. Nickase N.BspD6I cleaves one DNA strand only in double-stranded DNA and does not cleave single-stranded DNA. Site-specific methylase SscL1I (an isohypectomer of M*HinfI) that methylates adenine in the sequence 5'-GAGTC-3' prevents DNA hydrolysis by nickase BspD6I.