Isolation and partial characterization of plasmid DNA from Streptococcus thermophilus

Of 23 strains of Streptococcus thermophilus examined, 5 were found to contain a single small cryptic plasmid, designated pHM1 through pHM5. Through analysis by restriction endonuclease mapping and DNA-DNA hybridization, the five plasmids were found to be closely related. They were present in 4 to 18 copies per cell and ranged in size from 1.4 to 2.2 megadaltons. Plasmids pHM1 and pHM5, as well as pHM2 and pHM4, were found to be identical. Single restriction endonuclease sites were observed on each plasmid for PvuII and MboI. Plasmids pHM2 and pHM3 each had an additional single site for HhaI, and pHM1 had an additional single site for HindIII. The characteristics of these plasmids may make them useful for the development of cloning vectors for use in S. thermophilus.