| Abstract: | The replication region of the 2.77-kilobases (kb) plasmid pSTl from Streptococcus thermophilus No. 29 was identified. Deletion derivatives of pSTl were introduced into plasmid-free S. thermophilus No. 29 and examined for their ability to replicate autonomously. The nucleotide sequence had one open reading frame encoded for a 315 amino acid protein (Rep). Comparisons with proteins encoded by other Gram-positive bacterial plasmids strongly suggest that the deduced protein (RepS) encoded by pSTl has a replicative role. pSTl also contains a DNA sequence similar to the origin nick sequences of pLPl or pLAB1000, which initiate plasmid replication at the plus origin. In a maxicell system, E. coli CSR603 carrying pSTUC4 produced a protein that was considered to correspond to the product of the RepS gene. These results strongly suggest that pSTl is replicated following a rolling-circle mechanism via single-stranded DNA intermediates. |
