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The streptococcal plasmid pMV158 and its derivative pLSl are able to replicate and confer tetracycline resistance in both Gram-positive and Gram-negative bacteria. Copy numbers of pLSl were 24, 4 and 4 molecules per genome in Streptococcus pneumoniae, Bacillus subtilis and Escherichia coli, respectively. Replication of the streptococcal plasmids in E. coli required functional polA and recA genes. A copy-number mutation corresponding to a 332 base-pair deletion of pLSl doubled the plasmid copy number in all three species. Determination of the complete DNA sequence of pLSl revealed transcriptional and translational signals and four open reading frames. A putative inhibitory RNA was encoded in the region deleted by the copy-control mutation. Two putative mRNA transcripts encoded proteins for replication functions and tetracycline resistance, respectively. The repB gene encoded a trans-acting, 23,000 M …