Interspecific plasmid transfer between Streptococcus pneumoniae and Bacillus subtilis

Summary The streptococcal plasmids pMV158 and pLS1, grown in Streptococcus pneumoniae, were transferred to Bacillus subtilis by DNA-mediated transformation. The plasmids were unchanged in the new host; no deletions were observed in 80 instances of transfer. Their copy number was similar to that in S. pneumoniae. Two B. subtilis plasmids, pUB110 and pBD6, could not be transferred to S. pneumoniae. Hybrid plasmids were produced by recombining the EcoRI fragment of pBD6 that confers Km^r with EcoRI-cut pLS1, which confers Tc^r. The simple hybrid, pMP2, was transferable to both species and expressed Tc^r and Km^r in both. A derivative, pMP5, which contained an insertion in the pBD6 component, expressed a higher level of kanomycin resistance and was more easily selected in S. pneumoniae. Another derivative, pMP3, which contained an additional EcoRI fragment, presumably of pneumococcal chromosomal DNA, could not be transferred to B. subtilis. Previous findings that monomeric plasmid forms could transform S. pneumoniae but not B. subtilis were confirmed using single plasmid preparations. Although plasmids extracted from either species were readily transferred to S. pneumoniae, successive passage in B. subtilis increased the ability of plasmid extracts to transfer the plasmid to a B. subtilis recipient. This adaptation was tentatively ascribed to an enrichment of multimeric forms in extracts of B. subtilis as compared to S. pneumoniae. A review of host ranges exhibited by plasmids of Gram-positive bacteria suggested differences in their ability to use particular host replication functions. The pMP5 plasmid, with readily selectable Km^r and Tc^r markers in both hosts, and with the potential for inactivation of Km^r by insertion in the Bg/II site, could be a useful shuttle vector for cloning in S. pneumoniae and B. subtilis.