The 4.2-kb cryptic streptococcal plasmid, pVA380-1, has a broad host range and replicates via a rolling circle mode. The nucleotide base sequence of the 2.5-kb basic replicon of this plasmid, containing a plus origin, a minus origin, and a rep gene, was reported previously. The completed nucleotide base sequence of pVA380-1 revealed the presence of two additional open reading frames, ORF2 and ORF3. The protein predicted by ORF2 shared 74% amino acid identity with the Mob protein of the Group B streptococcal plasmid, pMV158. The 170 bases immediately 5' to ORF2 and to mob of pMV158 shared 87% homology and included a putative -10 region and a RSa site. A derivative of pVA380-1 into which a kanamycin resistance gene was inserted outside of the above 170 base region and ORF2 was mobilized by the conjugative streptococcal plasmid, pAM beta 1, into Enterococcus faecalis and Streptococcus sobrinus. Interruption of ORF2 by the same kanamycin resistance gene abolished the mobilizability of pVA380-1.