An inserted DNA fragment with plasmid features is uniquely associated with the presence of the GBSi1 group II intron in Streptococcus agalactiae

Abstract

The group II intron (GBSi1) identified downstream of the C5a-peptidase gene (scpB) in a subpopulation of Streptococcus agalactiae isolates is a suggested marker for a separate genetic lineage of serotype III isolates. In the present study two additional copies of GBSi1, one of which not previously described, were identified among serotype III isolates. All intron copies shared a common target site motif. A single copy of GBSi1 was found in a subgroup of serotype II and V isolates. In these isolates, the intron had inserted downstream of scpB, which suggests that this is the primary insertion site for GBSi1. Most bacterial group II introns described to date reside in transposable elements. The scpB locus was found to be flanked by insertion sequences similar to what has been described in an intronless serotype Ia isolate. However, this region contained an additional 2.1 kb DNA fragment present only in intron carrying isolates. This DNA fragment contained a partial transposase and putative plasmid related proteins. This may suggest that GBSi1 once was brought into the S. agalactiae genome by an integrated plasmid.