P1 and NR1 plasmid replication during the cell cycle of Escherichia coli

Replication patterns of the miniP1 plasmid pZC176, the miniNR1 plasmid pRR9 33, and the high copy miniNR1 derivative pRR942 were examined during the Es cherichia coli cell division cycle and compared to the cycle-specific repli cation pattern of a minichromosome and the cycle nonspecific pattern of pBR 322. In E. coli cells growing with doubling times of 40 and 60 min, the min iP1 plasmid was found to replicate with a slight periodicity during the div ision cycle. The periodicity was not nearly as pronounced as that of the mi nichromosome, was not affected by the presence of a minichromosome, and was not evident in cells growing more rapidly with a doubling time of min. Bot h miniNR1 plasmids, pRR933 and pRR942, replicated with patterns indistingui shable from that of pBR322 and clearly different from that of the minichrom osome. It is concluded that both P1 and NR1 plasmids can replicate at all s tages of the cell cycle but that pi displays a slight periodicity in replic ation prob ability in the cycle of slower growing cells. This periodicity d oes not appear to be coupled to a specific age in the cycle, but could be a ssociated with the achievement of a specifics cell mass per plasmid. During temperature shifts of a dnaC(Ts) mutant, the miniP1 plasmid and pBR322 rep licated with similar patterns that differed from that of the minichromosome , but were consistent with a brief eclipse between rounds of replication. ( C) 2001 Academic Press.