Single-strand DNA (ssDNA) was used to modify 10 nm nanogold to obtain an aptamer-modified nanogold resonance scattering (RS) probe (AussDNA) for detection of Hg(2+). In the presence of NaCl, Hg(2+) interacts with AussDNA to form very stable double-strand T-Hg(2+)-T mismatches and release nanogold particles that aggregate to large nanogold clusters causing the RS intensity at 540 nm to be enhanced linearly. On those grounds, 1.3-1667 nM Hg(2+) can be detected rapidly by the aptamer-modified nanogold RS assay, with a detection limit of 0.7 nM Hg(2+). If the large nanogold clusters were removed by membrane filtration, the excess AussDNA in the filtrate solution exhibits a catalytic effect on the new Cu(2)O particle reaction between NH(2)OH and Cu(2+)-EDTA complex at 60 degrees C. The excess AussDNA decreased with the addition of Hg(2+), which led the Cu(2)O particle RS intensity at 602 nm to decrease. The decreased RS intensity (DeltaI(602nm)) had a linear response to Hg(2+) concentration in the range of 0.1-400 nM, with a detection limit of 0.03 nM Hg(2+). This aptamer-modified nanogold catalytic RS method was applied for the detection of Hg(2+) in water samples, with sensitivity, selectivity, and simplicity.
Tens of thousands of chimeric RNAs have been reported. Most of them contain a short homologous sequence (SHS) at the joining site of the two partner genes but are not associated with a fusion gene. We hypothesize that many of these chimeras may be technical artifacts derived from SHS-caused mis-priming in reverse transcription (RT) or polymerase chain reactions (PCR). We cloned six chimeric complementary DNAs (cDNAs) formed by human mitochondrial (mt) 16S rRNA sequences at an SHS, which were similar to several expression sequence tags (ESTs).These chimeras, which could not be detected with cDNA protection assay, were likely formed because some regions of the 16S rRNA are reversely complementary to another region to form an SHS, which allows the downstream sequence to loop back and anneal at the SHS to prime the synthesis of its complementary strand, yielding a palindromic sequence that can form a hairpin-like structure.We identified a 16S rRNA that ended at the 4th nucleotide(nt) of the mt-tRNA-leu was dominant and thus should be the wild type. We also cloned a mouse Bcl2-Nek9 chimeric cDNA that contained a 5-nt unmatchable sequence between the two partners, contained two copies of the reverse primer in the same direction but did not contain the forward primer, making it unclear how this Bcl2-Nek9 was formed and amplified. Moreover, a cDNA was amplified because one primer has 4 nts matched to the template, suggesting that there may be many more artificial cDNAs than we have realized, because the nuclear and mt genomes have many more 4-nt than 5-nt or longer homologues. Altogether, the chimeric cDNAs we cloned are good examples suggesting that many cDNAs may be artifacts due to SHS-caused mis-priming and thus greater caution should be taken when new sequence is obtained from a technique involving DNA polymerization.
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