M5 and DL1 each express two related adhesin polypeptides, SspA and SspB, which are members of the antigen I/II family of streptococcal surface proteins. The and genes are tandemly arranged in both strains, with residing upstream of The genes are separated by approximately 400 nucleotides in DL1 and 1300 nucleotides in M5. The nucleotide sequence of the intergenic region of strain M5 is reported and the difference in length compared to DL1 shown to arise from the presence of an insertion sequence, designated IS consisting of 1197 bp. The nucleotide sequence of IS is highly homologous to IS of and is related to a lesser extent to other members of the IS family of bacterial insertion sequences. It contains a single ORF of 1026 bp, encoding a putative transposase polypeptide of 342 amino acids. The deduced transposase sequence exhibits 93% identity with the transposase polypeptides encoded by IS However, the protein lacks a 90 residue central domain that is present in the IS transposase and in the transposase polypeptides encoded by the related IS elements. In addition, the organization of the inverted repeats flanking the transposase gene in differs from IS Extension products generated from a -specific primer indicated that transcription initiates within the intergenic region in both strains, suggesting that and are independently transcribed. Transcription appears to initiate 42 bases upstream of in DL1 In contrast, transcription in M5 initiates at least 125 bases upstream of in close proximity to the terminal inverted repeat of IS These results indicate that the promoters of M5 and DL1 are not conserved and suggest that IS sequences may play a role in directing the expression of in M5.


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