IDENTIFICATION OF THE REPLICATION REGION OF STREPTOCOCCUS-THERMOPHILUS NO 29 PLASMID PST1

The replication region of the 2.77-kilobases (kb) plasmid pST1 from St reptococcus thermophilus No. 29 was identified. Deletion derivatives o f pST1 were introduced into plasmid-free S. thermophilus No. 29 and ex amined for their ability to replicate autonomously. The nucleotide seq uence had one open reading frame encoded for a 315 amino acid protein (Rep). Comparisons with proteins encoded by other Gram-positive bacter ial plasmids strongly suggest that the deduced protein (RepS) encoded by pST1 has a replicative role. pST1 also contains a DNA sequence simi lar to the origin nick sequences of pLP1 or pLAB1000, which initiate p lasmid replication at the plus origin. In a maxicell system, E. coli C SR603 carrying pSTUC4 produced a protein that was considered to corres pond to the product of the RepS gene. These results strongly suggest t hat pST1 is replicated following a rolling-circle mechanism via single -stranded DNA intermediates.