Biochemical studies of two Bacillus pumilus plasmids

Abstract

B. pumilus NRS 576 harbored an estimated two copies per chromosome of a covalently closed circular (CCC) DNA molecule, the 576 plasmid. The 576 plasmid has a buoyant density of 1.698 g/cm3 and a molecular weight of about 28 x 106. Plasmid copy number remained about the same in both exponentially growing and stationary phase cells. Spontaneous variants of NRS 576 that formed spores at an elevated frequency were designated as W mutants. W mutants appeared to have lost the 576 plasmid on the basis of the following: W mutants (38 tested) lacked detectable CCC DNA, and the majority of the plasmid homologous sequences in bulk NRS 576 DNA were absent from bulk W mutant DNA. B. pumilus ATCC 7065 harbored at least 10 copies per chromosome of a CCC DNA element, the 7065 plasmid. The 7065 plasmid has a buoyant density of 1.696 g/cm3 and a molecular weight of about 6 x 106. Although the copy number of the plasmid appeared to remain the same in exponentially growing and stationary phase cells, an additional CCC form of higher molecular weight was detected in stationary phase cells.