digest problem - extra bands (Jan/21/2005 )

I digested a plasmid with Xba 1 which should linearize it. However, aside from the 3kb (supercoiled) and 5kb(linearized) band I see 4kb, 6kb, 7kb and other large bands. Digestions with other enzymes give expected results. What's going on?

Are there genome DNA in your plasmid solution?

In which buffer is the supercoiled plasmid you ran? Maybe the Xba digestion didn't work and these large bands you see are just the undigested supercoiled plasmid in the salt buffer used for the digestion.