Abstract
The origin of lagging strand synthesis in pUB110, oriL, has been localized within 140 bases outside the pUB110 minimal replicon. The oriL DNA sequence is a cis-acting and orientation dependent determinant required for normal plasmid replication. Rearrangements affecting oriL cause plasmid instability, lead to the accumulation of replication intermediates and result in a marked reduction of the plasmid copy number in some recombination deficient mutant strains. In addition, deletion of oriL triggers a dnaB-dependent mode of replication. Insertion of the functionally asymmetric oriL region in the proper orientation into pC194 reduces the accumulation of single-stranded DNA during the replication of this plasmid.