Characterisation of cryptic plasmid pPG01 from Propionibacterium granulosum, the first plasmid to be isolated from a member of the cutaneous propionibacteria

A cryptic plasmid, pPG01 (3539 bp), was isolated from Propionibacterium granulosum and sequenced. Analysis of open reading frames (ORFs) predicted pPG01 to encode three proteins. The largest protein (447 amino acids) showed homology to the FtsK/SpoIIIE family of proteins involved in chromosome partitioning during cell division and conjugal transfer of DNA. A second protein of 433 amino acids showed homology to plasmid replication proteins that mediate replication by the rolling circle mechanism. A third protein of 124 amino acids had no predicted function. All three ORFs were expressed as shown by reverse transcription-PCR analysis. Putative double- and single-stranded origins of replication were identified. Rolling circle replication of pPG01 was confirmed by the detection of single-stranded DNA intermediates. The first characterisation of a plasmid from the cutaneous propionibacteria may lead to development of a vector system to enable the genetic manipulation of this important group of organisms.