In preparing plasmid DNA, make sure that the RNase used for purification is not contaminated in the final purified DNA.

Using a filter-based purification kit such as QIAprep Spin Miniprep Kit (Qiagen) or Wizard Plus SV Minipreps DNA Purification System (Promega), for example, results in contamination of RNase A (contained in Lysis buffer) in to the final purified DNA solution. If you add this solution to the PUREfrex^® reaction mixture as template DNA without further treatment, transcription products and other RNAs in the reaction mixture are digested, which inhibit protein synthesis.

Therefore, treat the DNA with phenol/chloroform to denature RNase and then re-purify the DNA by ethanol precipitation to prepare it with free of RNase activity. Adding RNase inhibitor to the PUREfrex^® reaction mixture is also effective. Plasmid DNAs purified with Plasmid Mini Kit (Qiagen) has minimal contamination with RNase because the plasmid DNA eluted from resin is precipitated by adding isopropanol. Plasmid DNA purified with this kit are verified to use without any treatment.　
You can find the example with plasmid in the link here.