Plasmids with pMB1 replication origin are likely of low copy number in Bacillus subtilis

Origin of replication of plasmid is correlated with plasmid copy number in a cell. Specifically, some replication origin generally lead to low copy number, while others are correlated with high copy number. More importantly, an origin of replication that leads to high copy number in one species may only generate low copy number in another species. Personal experience with the transformation of plasmid with pMB1 replication origin into natural competent Bacillus subtilis suggested that the plasmid could only replicate to low copy number in the cell. This analysis draws from observations of inability of the transformed B. subtilis to survive under high concentration of ampicillin, which is the antibiotic resistance marker on the plasmid. Such a situation suggests that the cell did not have sufficient amount of enzyme to ameliorate the effects of ampicillin to allow the cell to grow under relatively high antibiotic stress, which meant that the plasmid encoding the antibiotic resistance gene was of low copy number. Reduction in the antibiotic concentration in the growth medium finally allowed transformed B. subtilis to grow to sufficient cell density. In contrast, Escherichia coli BL21 (DE3) transformed with the same plasmid could survive under high ampicillin stress, which suggested that the plasmid replicated to high copy number in the species. Overall, the presented observations suggest that plasmid with pMB1 origin of replication would result in low copy number in B. subtilis, which highlights that plasmids with the same replication origin need not be replicated to similar copy number in different bacterial species.