INTERRUPTION OF THE STREPTOCOCCUS-GORDONII M5 SSPA SSPB INTERGENIC REGION BY AN INSERTION-SEQUENCE RELATED TO IS1167 OF STREPTOCOCCUS-PNEUMONIAE/

Streptococcus gordonii M5 and DL1 each express two related adhesin pol ypeptides, SspA and SspB, which are members of the antigen I/II family of streptococcal surface proteins. The sspA and sspB genes are tandem ly arranged in both strains, with sspA residing upstream of sspB. The genes are separated by approximately 400 nucleotides in S. gordonii DL 1 and 1300 nucleotides in S. gordonii M5. The nucleotide sequence of t he sspA/sspB intergenic region of strain M5 is reported and the differ ence in length compared to S. gordonii DL1 shown to arise from the pre sence of an insertion sequence, designated ISSg1, consisting of 1197 b p. The nucleotide sequence of ISSg1 is highly homologous to IS1167 of Streptococcus pneumoniae and is related to a lesser extent to other me mbers of the IS1096 family of bacterial insertion sequences. It contai ns a single ORF of 1026 bp, encoding a putative transposase polypeptid e of 342 amino acids. The deduced transposase sequence exhibits 93 % i dentity with the transposase polypeptides encoded by IS1167. However, the S. gordonii protein lacks a 90 residue central domain that is pres ent in the IS1167 transposase and in the transposase polypeptides enco ded by the related IS elements. In addition, the organization of the i nverted repeats flanking the transposase gene in S. gordonii differs f rom IS1167. Extension products generated from a sspB-specific primer i ndicated that transcription initiates within the intergenic region in both S. gordonii strains, suggesting that sspA and sspB are independen tly transcribed. Transcription appears to initiate 42 bases upstream o f sspB in S. gordonii DL1. In contrast, sspB transcription in M5 initi ates at least 125 bases upstream of sspB, in close proximity to the te rminal inverted repeat of ISSg1. These results indicate that the sspB promoters of S. gordonii M5 and DL1 are not conserved and suggest that ISSg1 sequences may play a role in directing the expression of sspB i n S. gordonii M5.