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The structure of SgrAI bound to DNA; recognition of an 8 base pair target

The three-dimensional X-ray crystal structure of the ‘rare cutting’ type II restriction endonuclease SgrAI bound to cognate DNA is presented. SgrAI forms a dimer bound to one duplex of DNA. Two Ca2+ bind in the enzyme active site, with one ion at the interface between the protein and DNA, and the second bound distal from the DNA. These sites are differentially occupied by Mn2+, with strong binding at the protein–DNA interface, but only partial occupancy of the distal site. The DNA remains uncleaved in the structures from crystals grown in the presence of either divalent cation. The structure of the dimer of SgrAI is similar to those of Cfr10I, Bse634I and NgoMIV, however no tetrameric structure of SgrAI is observed. DNA contacts to the central CCGG base pairs of the SgrAI canonical target sequence (CR|CCGGYG, | marks the site of cleavage) are found to be very similar to those in the NgoMIV/DNA structure (target sequence G|CCGGC). Specificity at the degenerate YR base pairs of the SgrAI sequence may occur via indirect readout using DNA distortion. Recognition of the outer GC base pairs occurs through a single contact to the G from an arginine side chain located in a region unique to SgrAI

Revised UV extinction coefficients for nucleoside-5′-monophosphates and unpaired DNA and RNA

Ultraviolet absorption provides the nearly universal basis for determining concentrations of nucleic acids. Values for the UV extinction coefficients of DNA and RNA rely on the mononucleotide values determined 30–50 years ago. We show that nearly all of the previously published extinction coefficients for the nucleoside-5′-monophosphates are too large, and in error by as much as 7%. Concentrations based on complete hydrolysis and the older set of values are too low by ∼4% for typical RNA and 2–3% for typical DNA samples. We also analyzed data in the literature for the extinction coefficients of unpaired DNA oligomers. Robust prediction of concentrations can be made using 38 µg/A(260) unit for single-stranded DNA (ssDNA) having non-repetitive sequences and 40–80% GC. This is superior to currently used predictions that account for nearest-neighbor frequency or base composition. The latter result in concentrations that are 10–30% too low for typical ssDNA used as primers for PCR and other similar techniques. Methods are described here to accurately measure concentrations of nucleotides by nuclear magnetic resonance. NMR can be used to accurately determine concentrations (and extinction coefficients) of biomolecules within 1%