[Objectives] Mobile genetic elements (MGEs) are known to be involved in a number of genetic events in bacteria. However, there are few reports on MGEs in Streptococcus anginosus subsp. anginosus (SAA), a commensal strain in the human oral cavity. Recently, in our search for MGEs in SAA clinical isolates, we discovered a new plasmid in strain 0430-08. In the present study, we examined the genetic structure and characteristics of this novel plasmid.
[Methods] The nucleotide sequence of the purified plasmid from strain 0430-08 (pSAA0430-08) was determined and the annotation of its predicted ORFs was performed. Studies were carried out to identify the function and participation of translational products of the predicted ORFs in plasmid replication and maintenance.
[Results and Discussion] We found that pSAA0430-08 is a circular plasmid with 7,038 bp and ~31% G/C content, and composed of 10 ORFs including ORFs encoding a putative toxin-antitoxin system. In addition, pSAA0430-08 contains two important elements suggested to function during replication: ORF1 was predicted to encode the replication protein RepB homolog, and iteron. Because none of the transformants carrying variant plasmid constructs lacked ORF1 or iteron, we conclude that both of these are essential elements for pSAA0430-08 replication. In addition, the stable maintenance of this plasmid in the SAA strain is thought to be due to the presence of the toxin-antitoxin system found on the plasmid. The characteristics of the remaining ORFs with unknown function identified on the plasmid are currently under investigation.