Cosmid isolation

[Dubear Kroening]

We have recently come across an unusual result when isolating cosmid DNA.  It 
seems as if there are extra bands, one just barely running out of the well on 
an agarose gel.  It may be due to nicking, and we have noticed that 
phenol/chloroform extraction reduces some of the extra bands.  The real 
unusual part is that there is one cosmid that when isolated yields only a 
single band.  The isolation has been tried from two different strains of E. 
coli, using a Qiagen midi-prep or alkaline lysis with no notable differences.  
Any ideas?

Also, I though I had read an article that used exonuclease to help get rid of 
chromosomal DNA in a plasmid/cosmid prep.  I searched BioTechniques, but 
couldn't find the article.  Has anyone else seen this.

Dubear Kroening
dubear at whitewater.chem.wisc.edu