Sequencing and characterization of pST1, a cryptic plasmid from Streptococcus thermophilus

Eighty-six strains of S. thermophilus were examined for their plasmid content. Thirteen strains were found to contain one or two plasmids ranging in size from 2.1 to 7.4 kb. DNA-DNA hybridization analysis revealed the presence of five distinct groups of DNA homology. The complete nucleotide sequence of plasmid pST1 (Accession number X65856), which belongs to the major homology group, was determined. It has a molecular size of 2093 bp, a GC content of 35% and contains one major open reading frame of 945 bp (ORF A). The predicted protein, designated Rep A, showed sequence homology with replication proteins from a group of plasmids which are known to replicate via single-stranded DNA intermediates (ssDNA plasmids).