Nucleic acid purification

1. A process for purifying plasmid DNA from a nucleic acid-containing sample comprising plasmid DNA and contaminants including RNA, which process comprises a step of contaminant removal, comprising:

(a) treating the nucleic acid-containing sample to form a nucleic acid solution having a concentration of monovalent cations selected from the group consisting of sodium and potassium, which nucleic acid solution includes the RNA;

(b) contacting the nucleic acid solution comprising the plasmid DNA and RNA with an ultrafiltration membrane having a molecular weight exclusion limit of at least 30 kDa and no more than 100 kDa, which ultrafiltration membrane is provided in a screen-channel ultrafiltration unit which generates turbulent sample flow, whereby turbulence is provided in the region of the ultrafiltration membrane, wherein the step of contacting includes a step of diafiltration and wherein the step of contacting and the step of diafiltering the nucleic acid solution with the ultrafiltration membrane takes place in the presence of a concentration of the monovalent cations which is at least about 0.35M but does not exceed about 2M for a time sufficient to remove substantially all RNA and form a retentate containing plasmid DNA; and

(c) collecting the retentate.

2. A process according to claim 1 , wherein the molecular weight exclusion limit of the ultrafiltration membrane is at least 50 kDa and no more than 100 kDa.

3. A process according to claim 1 , wherein at least step (b) is performed at a temperature in the range 30° C. to 60° C.

4. A process according to claim 1 , which further comprises a step (d) of subjecting the retentate to further purification.

5. A process according to claim 4 , wherein step (d) comprises: (i) contacting the retentate with an anion exchange resin under conditions to bind plasmid DNA; (ii) optionally washing the resin to remove impurities from the plasmid DNA; and (iii) eluting the plasmid DNA.

6. A process according to claim 4 , wherein the retentate is contacted with a hydrophobic solid phase under conditions to separate contaminants from plasmid DNA.

7. A process according to claim 1 , wherein the nucleic acid-containing sample is prepared from a crude nucleic acid solution containing plasmid DNA and RNA by (i) treating the crude nucleic acid solution to provide a calcium ion concentration sufficient to precipitate a majority of the RNA; and (ii) separating the liquid phase therefrom, the liquid phase comprising the nucleic acid-containing sample.

8. A process according to claim 7 , wherein the calcium ion concentration is in the range 0.1 to 0.3M.

9. A process according to claim 7 , wherein the crude nucleic acid solution is prepared by (i) providing a cell free extract comprising plasmid DNA and RNA; and (ii) concentrating the extract by ultrafiltration.

10. A process according to claim 9 , wherein at least step (ii) is performed at a temperature in the range 30° C. to 60° C.

11. A process according to claim 9 , which further comprises a step of diafiltration against a solution suitable for use in the subsequent step of treating the solution to provide a calcium ion concentration sufficient to precipitate the majority of RNA.

12. A process according to claim 9 , wherein the ultrafiltration is carried out in the presence of a detergent.

13. A process according to claim 12 , wherein the detergent comprises sodium dodecylsulphate.

14. A process for plasmid DNA purification from a nucleic acid containing sample comprising plasmid DNA and contaminants including RNA, which process comprises a step of contaminant removal, comprising:

(a) treating the nucleic acid containing sample to form a nucleic acid solution having a concentration of monovalent cations selected from the group consisting of sodium and potassium, which nucleic acid solution includes the RNA;

(b) contacting the nucleic acid solution comprising the plasmid DNA and RNA with an ultrafiltration membrane having a molecular weight exclusion limit of at least 30 kDa and no more than 100 kDa, which ultrafiltration membrane is provided in a screen-channel ultrafiltration unit which generates turbulent sample flow, whereby turbulence is provided in the region of the ultrafiltration membrane, wherein the step of contacting includes a step of diafiltration and wherein the step of contacting and the step of diafiltering the nucleic acid solution with the ultrafiltration membrane takes place in the presence of a concentration of the monovalent cations which is at least about 0.35M but does not exceed about 2M for a time sufficient to remove all RNA and form a retentate containing plasmid DNA;

(c) collecting the retentate;

(d) subjecting the retentate to hydrophobic chromatography;

(e) contacting the retentate with an anion exchange resin under conditions to bind plasmid DNA;

(f) optionally washing the resin to remove impurities from the plasmid DNA; and

(g) eluting the plasmid DNA.

15. A process for purifying plasmid DNA from a nucleic acid-containing sample comprising plasmid DNA and contaminants including RNA, which process comprises a step of contaminant removal, comprising:

(a) treating the nucleic acid-containing sample to form a nucleic acid solution having a concentration of monovalent cations selected from the group consisting of sodium and potassium, which nucleic acid solution includes the RNA;

(b) contacting the nucleic acid solution comprising the plasmid DNA and RNA with an ultrafiltration membrane having a molecular weight exclusion limit of at least 30 kDa and no more than 100 kDa, which ultrafiltration membrane is provided in a screen-channel ultrafiltration unit which generates turbulent sample flow, whereby turbulence is provided in the region of the ultrafiltration membrane, wherein the step of contacting includes a step of diafiltration and wherein the step of contacting and the step of diafiltering the nucleic acid solution with the ultrafiltration membrane takes place in the presence of a concentration of the monovalent cations which is at least about 0.35M but does not exceed about 2M for a time sufficient to remove substantially all RNA and form a retentate containing plasmid DNA; and

(c) collecting the retentate,

wherein the nucleic acid solution is contacted with the ultrafiltration membrane under pressure and at a temperature range of room temperature to 60° C.

16. A process for purifying plasmid DNA from a nucleic acid-containing sample comprising plasmid DNA and contaminants including RNA, which process comprises a step of contaminant removal, comprising:

(a) treating the nucleic acid-containing sample to form a nucleic acid solution having a concentration of monovalent cations selected from the group consisting of sodium and potassium, which nucleic acid solution includes the RNA;

(b) contacting the nucleic acid solution comprising the plasmid DNA and RNA with an ultrafiltration membrane having a molecular weight exclusion limit of at least 30 kDa and no more than 100 kDa, which ultrafiltration membrane is provided in a screen-channel ultrafiltration unit which generates turbulent sample flow, whereby turbulence is provided in the region of the ultrafiltration membrane, wherein the step of contacting includes a step of diafiltration and wherein the step of contacting and the step of diafiltering the nucleic acid solution with the ultrafiltration membrane takes place in the presence of a concentration of the monovalent cations which is at least about 0.35M but does not exceed about 2M for a time sufficient to remove substantially all RNA and form a retentate containing plasmid DNA; and

(c) collecting the retentate,

wherein the nucleic acid solution is contacted with the ultrafiltration membrane under a pressure range of 15 to 30 psi and at a temperature range of room temperature to 60° C.

17. A process for purifying plasmid DNA from a nucleic acid-containing sample comprising plasmid DNA and contaminants including RNA, which process comprises a step of contaminant removal, comprising:

(a) treating the nucleic acid-containing sample to form a nucleic acid solution having a concentration of monovalent cations selected from the group consisting of sodium and potassium, which nucleic acid solution includes the RNA;

(b) contacting the nucleic acid solution comprising the plasmid DNA and RNA with an ultrafiltration membrane having a molecular weight exclusion limit of at least 30 kDa and no more than 100 kDa, which ultrafiltration membrane is provided in a screen-channel ultrafiltration unit which generates turbulent sample flow, whereby turbulence is provided in the region of the ultrafiltration membrane, wherein the step of contacting includes a step of diafiltration and wherein the step of contacting and the step of diafiltering the nucleic acid solution with the ultrafiltration membrane takes place in the presence of a concentration of the monovalent cations which is at least about 0.35M but does not exceed about 2M for a time sufficient to remove RNA and form a retentate containing plasmid DNA; and

(c) collecting the retentate,

wherein the nucleic acid solution is contacted with the ultrafiltration membrane under a pressure range of 15 to 30 psi and a temperature range of room temperature to 60° C.

18. A process for purifying plasmid DNA from a nucleic acid-containing sample comprising plasmid DNA and contaminants including RNA, which process comprises a step of contaminant removal, comprising:

(a) treating the nucleic acid-containing sample to form a nucleic acid solution having a concentration of monovalent cations selected from the group consisting of sodium and potassium, which nucleic acid solution includes the RNA;

(b) contacting the nucleic acid solution comprising the plasmid DNA and RNA with an ultrafiltration membrane having a molecular weight exclusion limit of at least 30 kDa and no more than 100 kDa, which ultrafiltration membrane is provided in a screen-channel ultrafiltration unit including a separator positioned between a pair of membranes which generates turbulent sample flow, whereby turbulence is provided in the region of the ultrafiltration membrane, wherein the step of contacting includes a step of diafiltration and wherein the step of contacting and the step of diafiltering the nucleic acid solution with the ultrafiltration membrane takes place in the presence of a concentration of the monovalent cations which is at least about 0.35M but does not exceed about 2M for a time sufficient to remove RNA and form a retentate containing plasmid DNA; and

(c) collecting the retentate,

wherein the nucleic acid solution is contacted with the ultrafiltration membrane under a pressure range of 15 to 30 psi and a temperature range of room temperature to 60° C.