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Duplex-specific nuclease
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Duplex-specific nuclease (DSN) is an enzyme purified from hepatopancreas of Red King (Kamchatka) crab (Shagin et al., 2002). DSN shows a strong preference for cleaving double-stranded (ds) DNA and DNA in DNA-RNA hybrid duplexes, and is practically inactive towards single-stranded (ss) DNA or single or double-stranded RNA. Moreover, the enzyme is capable to discriminate between perfectly and nonperfectly matched short DNA duplexes. DSN is widely used for full-length enriched cDNA normalization (Zhulidov et al., 2004; Bogdanova et al., 2011a) and for construction of normalized RNA-seq libraries for next generation sequencing ((Christodoulou et al., 2011), Illumina DSN normalization protocol). The list of approved DSN applications also includes normalization of genomic DNA (Shagina et al., 2010), cDNA depletion and ribosomal cDNA depletion (Bogdanova et al., 2009; Bogdanova et al., 2011b; Yi et al., 2011), cDNA subtraction (Peng et al., 2008), SNP detection (Shagin et al., 2002; Liu et al., 2011), construction of repeat-free FISH probes (Swennenhuis et al., 2012), multiplexed fluorescence detection of miRNAs (Yin et al., 2012), quantitative telomeric overhang determination (Zhao et al., 2008; Zhao et al., 2011), etc. |
Substrate specificity: DSN exhibits strong cleavage preference for ds DNA substrates. It can show minor activity against ss DNA when both DSN enzyme and substrate are present at high concentrations, however this activity is not detectable in presence of competitive ds DNA and RNA (Zhao et al., 2008). DSN does not cleave RNA, but effectively cleaves DNA in DNA-RNA hybrid duplexes (Shagin et al., 2002)
Action of DSN on ss DNA of phage M13 and ds DNA of phage λ.Lanes 1, 2 – negative controls, incubation without nuclease. Lane 1: phage M13 DNA alone, Lane 2 – mixture containing phage M13 and λ DNA. Lanes 3; 4 – digestion of phage M13 and λ DNA mixture by DSN at 70°C for 1.5 min (lane 3) and 5 min (lane 4). |
Analysis of DSN activity on synthetic oligonucleotide substrates revealed that the enzyme discriminates between perfectly matched short DNA-DNA duplexes (10-12 bp) and duplexes of the same length with at least one mismatch. It requires at least 10 bp DNA or 15 bp DNA-RNA perfect duplex for cleavage.
DSN action on one mismatch-containing and perfectly matched DNA duplexes.Duplexes formed by 5-carboxyfluorescein (Fl)-5'-gccctatagt-3'-TAMRA signal probe and complementary targets (5’-actcactataCggcgaat-3’ and 5’-actcactatagggcgaat-3’) were incubated with DSN at 35°C for 15 min. Emission spectra were obtained on the spectrofluorimeter, with excitation at 480 nm. Dotted line – substrate fluorescence in the absence of enzyme; firm line – substrate fluorescence after incubation with DSN. |
DSN acquires its enzymatic activity in the presence of divalent cations (Mn2+, Co2+, or Mg2+). Mg2+ ion concentration for most applications should be at least 5 mM. DSN is inhibited by EDTA.
The temperature optimum for DSN activity is 60°C. However, already at 80°C DSN retains only 10% of activity. This sharp decrease in activity may be attributable, at least in part, to ds DNA substrate denaturation.
The optimal pH for DSN activity is 6.6. At pH values between 3 and 5, DSN displays only 10% of its maximal activity. The nuclease is stable at a wide range of pH (from 4 to 12) and temperatures below 65°C. About 60% of DSN activity remains after 30-min incubation at 70°C, and 40% – after incubation at 80°C.
Effects of various conditions on DSN activity.Activity of DNAse on ds DNA substrate was measured using modified Kunitz assay (Kunitz, 1950) |
Incubation of DSN with aggressive chemicals like 1% SDS, 10 mM β-mercaptoethanol, and 0.3% hydrogen peroxide at 37°C results in only a moderate drop in activity, after 30 min incubation about 90% of activity is preserved. However, a sharp decrease in activity was observed upon chemical treatment at 60°C. SDS completely inhibits DSN activity, while β-mercaptoethanol and hydrogen peroxide induce approximately 70% and 80% loss in activity, respectively.
DSN is highly sensitive to ionic force (e.g., a 10 times decrease in catalytic activity is observed in the presence of 0.2 M NaCl). The addition of chaotropic agents or polyamines to the reaction mixture also results in suppression of enzyme activity.
DSN is tolerant to proteinase K treatment (incubation at 37°C for 30 min).
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