Mutually Exclusive Distribution of IS 1548 and GBSi1, an Active Group II Intron Identified in Human Isolates of Group B Streptococci

Abstract

The present study shows that active, self-splicing group II intron GBSi1 is located downstream of the C5a-peptidase gene, scpB, in some group B streptococcus (GBS) isolates that lack insertion sequence IS1548. IS1548 was previously reported to be often present at the scpB locus in GBS isolated in association with endocarditis. Since none of 67 GBS isolates examined, 40 of which were of serotype III, harbored both IS1548 and GBSi1, these two elements are suggested to be markers for different genetic lineages in GBS serotype III. The DNA region downstream of scpB in GBS isolates harboring either GBSi1, IS1548, or none of these mobile elements was found to encode the laminin binding protein, Lmb, which shows sequence similarities to a family of streptococcal adhesins. IS1548 is inserted 9 bp upstream of the putative promoter for lmb, while the insertion site for GBSi1 is located 88 bp further upstream. Sequences highly similar to GBSi1 exist also in Streptococcus pneumoniae. An inverted repeat sequence, with features typical of transcription terminators, was identified immediately upstream of the insertion site for the group II intron both in the GBS and S. pneumoniae sequences. This motif is suggested to constitute a target for the GBS intron as well as for rather closely related introns in Bacillus halodurans, Pseudomonas alcaligenes, and Pseudomonas putida. When transcripts containing the GBSi1 intron were incubated at high concentrations of ammonium and magnesium, a major product with the expected length and sequence for the ligated exons was generated. Unlike, however, all members of group II investigated so far, the excised intron was in linear, rather than in a branched (lariat), form.

Physiological impact of transposable elements encoding DDE transposases in the environmental adaptation of Streptococcus agalactiae

“…These contradictory reports point out that the exact involvement of HylB in systemic infections needs further investigations (Bohnsack et al, 2001;Musser et al, 1989;Rojo et al, 2008). IS1548 possesses another preferential insertion target just upstream of the lmb gene encoding a bifunctional laminin-binding protein/zinc-binding protein (Granlund et al, 1998(Granlund et al, , 2001; Fig. 1).…”

“…We also identified an IS1548 insertion in a region very close to and upstream of adcA, encoding the high affinity zinc-binding protein of the Adc transporter (Fléchard et al, 2013b). Moreover, the laminin-binding protein Lmb, encoded by a gene that is one of the preferential targets of IS1548 in S. agalactiae, exhibits significant homologies to AdcA, and was shown to bind a zinc divalent cation in S. agalactiae (Granlund et al, 1998(Granlund et al, , 2001Ragunathan et al, 2009). Furthermore, IS1548 insertions identified within the adcRCB operon spontaneously appeared during growth in vitro, suggesting, as is the case with covS, that a bacterial subpopulation carrying the mutation could emerge during the infectious process (Fléchard et al, 2013a).…”

Insertion of group II intron retroelements after intrinsic transcriptional terminators

“…In contrast to such site-specificity, introns of the phylogenetic subclass ''bacterial class C'' instead are located after intrinsic transcriptional terminators (stem-loop followed by a run of Ts) (15)(16)(17). In some genomes, an intron copy is found in multiple sites throughout a genome, after nonidentical terminators (15), suggesting that a motif is targeted rather than a discrete homing site.…”

Recurrent insertion of 5′-terminal nucleotides and loss of the branchpoint motif in lineages of group II introns inserted in mitochondrial preribosomal RNAs

“…The EBS2-IBS2 pairing is present in a majority of group II introns, with the exception of members of subgroup IIC, whose 59 exon displays a hairpin structure at the expected location for the IBS2 sequence (Granlund et al 2001;Quiroga et al 2008). What has been lost, in fact, is not only the EBS2 loop, but an entire subdomain that, in subgroup IIB, branches off the 59 strand of the stem connecting the internal loops that contain the EBS3 and a9 nucleotides.…”

“…3B). This would not be without precedent, for the only intron subclass-subgroup IIC-in which these components are systematically missing is noteworthy for (most of) its members initiating selfsplicing in vitro by hydrolysis (Granlund et al 2001;Toor et al 2006). Unfortunately, possible ways in which EBS2 and the structures that surround it might affect the balance between transesterification and hydrolysis remain difficult to think of at present.…”