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Dear All: This is off topic but I would rather try here first. I am wondering whether Dnase could be a contaminant during the nickel affinity purification of a his.tag protein expressed using pET29b. The cells were disrupted using sonication only. Very high yield (30 mg/liter of cell culture). FPLC purification. SDS-PAGE shows a singe band even when overloaded. Silver staining is in progress. The protein I am interested in does not have any similarity to Dnase. However, when treated with supercoiled dna, the dna is degraded as efficiently as with Dnase I from Sigma. I am looking for ways to show that my preparation does not have any Dnase contamination. I would appreciate it very much if someone points me in the right direction. Thanks a lot Subbu