Selective Precipitation of DNA by Spermine during the Chemical Extraction of Insoluble Cytoplasmic Protein

The direct chemical extraction of recombinant LI protein (the major capsid protein of human papillomavirus type 16) from the cytoplasm of E, coli HMS174(DE3) has recently been demonstrated at high cell density (to OD600 = 160) without the use of reducing agent (J). Coextraction of DNA at high concentration prevents direct coupling to postextraction recovery operations including expanded bed adsorption. In this study, spermine is used to selectively precipitate DNA during chemical extraction. Highly efficient and selective DNA precipitation was achieved. An approximate IO-fold increase in the specific spermine concentration (mg of spermine/mg of DNA) was required to precipitate DNA when 8 M urea was added to the extraction buffer. EDTA (3 mM), required for effective chemical extraction, does not significantly inhibit DNA precipitation. Precipitation selectivity was demonstrated in a bovine serum albumin spiking test, with almost complete recovery of the spiked protein. During studies on the direct extraction of LI protein from cells at OD600 = 80, high DNA removal efficiency (>85%) and negligible LI protein coprecipitation were achieved. This selective precipitation technique simply requires the addition of spermine to the chemical extraction buffer and therefore does not increase technique complexity. This modification enhances the methodʹs general applicability and enables direct coupling to downstream recovery units following chemical extraction at high cell and product concentrations.