Evidence for a nonrandom base sequence in a Bacillus pumilus plasmid: EcoR1 endonuclease digestion of pPL576

EcoR1 endonuclease digested the Bacillus pumilus plasmid pPL576 (molecular weight similar to 28 X 10-6) into three distinct size classes of linear fragments. The molecular weights of the fragments are 13.0 X 10-6, 0.5 X 10-6, and 6.5 X 10-6 by sucrose gradient analysis. By electron microscope analysis the three fragments account for about 99% of the intact plasmid. These results indicate that pPL576 molecules contain a nonrandom base sequence, and are consistent with the interpretation that pPL576 is autonomous and not the result of cyclization of random chromosome fragments.