A functional lagging strand origin does not stabilize plasmid pMV158 inheritance in Escherichia coli

Plasmid rolling circle replication generates single-stranded DNA intermedia tes. The intracellular amount of these molecules depends upon the efficienc y of the conversion of single-stranded into double-stranded plasmid forms, that is, the functionality of the lagging strand origin (sso). The broad-ho st-range streptococcal plasmid pMV158 harbors two different ssos, both of w hich function efficiently in Streptococcus pneumoniae but poorly in Escheri chia coli. Plasmid pMV158 is stably inherited in the pneumococcal host, but it is unstable in E. coli. A pMV158 derivative lacking its two ssos is uns table in both strains, We have cloned into this derivative the coliphage fl lagging strand origin. Whereas the fl sso was fully functional in E. coli, it did not show any activity in S. pneumoniae, a bacteria closely related to the pMV158 natural host. The presence of the fl sso did not stabilize pM V158 inheritance in either the gram-positive or the gram-negative host. (C) 2000 Academic Press.