LAGGING-STRAND ORIGINS OF THE PROMISCUOUS PLASMID PMV158 - PHYSICAL AND FUNCTIONAL-CHARACTERIZATION

The streptococcal plasmid pMV158 replicates by a rolling circle mechan ism. which involves the generation of single-stranded plasmid DNA inte rmediates. This plasmid has the unique feature of having two lagging-s trand origins of replication. One of these origins, termed ssoU, is fu nctional in Streptococcus pnemoniae and in Bacillus subtilis in an ori entation-dependent manner. The other origin, ssoA, is only functional in the former host. RNA polymerase seems to be involved in the initiat ion of the conversion of single- to double-stranded plasmid DNA from b oth ssoA and ssoU. Mutational and deletion analyses have allowed us to define ssoA as being within a highly structured, nan-coding 199 bp re gion. Within this region, two elements which are conserved in several rolling-circle replicating plasmids are located, the recombination sit e RS(B) and a 6 base consensus sequence. Both elements may play a role in the conversion of single- to double-stranded plasmid DNA.