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Removing traces of DNA ("RNAse-free" isn't always accurate!)

Michael Clark michael-clark at uiowa.edu
Thu Dec 21 18:32:46 EST 1995

There have been several posts regarding the use of RNAse free DNAse to
clean up in vitro transcribed RNA prior to RNAse protection or
competitive RT-PCR.

I just thought I should share by (bad) experience with anyone doing
this.  Not all products sold as "RNAse free" really are.

Several months ago, I needed to remove DNA from total RNA preps. I used
Gibco's "RNAse-free DNAse I" as per their published protocol.  My RNA
kept getting degraded.  I went through all of my solutions (I had never
had problems with RNA degradation before, despite doing a lot of work
with it), but all of these were fine.  So I checked the DNAse by
incubating it with a RNA sample of known purity.  Guess what, the RNAse
free DNAse wasn't.  It ate my RNA (several micrograms) in under 5

Figuring I (or someone else) may have contaminated the DNAse, I
carefully opened a brand new sealed package.  It was full of RNAses
too.  When I called Gibco, they gave me the run around; suggesting I
use a different (and more expensive) DNAse, despite the fact that the
product I was using was *specifically* labeled as being suitable for
the purpose I was using it for.

Since then, I switched to Promega RQ DNAse and haven't had a problem
since.  It works great, and I've had *no* RNAse contamination problems.

Although we use other Gibco products with no problems, I'd stay away
from Gibco "RNAse free DNAse."  It wasn't RNAse free and I was not
satisfied with their handling of the problem.  I lost very valuble
samples and time.  I won't use their DNAse again.

Michael Clark
michael-clark at uiowa.edu
University of Iowa
Molecular Biology Program

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