Lambda DNA resists digestion

[Rob Mauk]

I have isolated lambda gt10 DNA by PEG precipitation followed by organic 
extraction and ethanol precipitation, and find that it is almost 
completely resistant to digestion by EcoRI and all other enzymes that I 
have tested.  Proteinase K treatment did not help, nor did additional 
extractions and precipitations.  Plasmid DNA digests normally with these 
enzymes, but if the plasmid is mixed with the lambda DNA, it is rendered 
undigestable.  Any comments or suggestions regarding the nature of this 
contaminant, or solutions to the problem would be greatly appreciated.  
Thank you.