Insertion Sequence elements in Streptococcus mutans

IS elements consist of DNA encoding only the transposase necessary for their own mobilisation. Approximately 25% of S. mutans isolates carry IS3 and genome sequencing of S. mutans UA159 revealed 5 intact IS3 elements. Objectives: To investigate whether the pattern of distribution of IS3 elements in strain UA159 is replicated in other strains and to determine IS3 activity. Methods: Using UA159 genome sequence, PCR primers were designed to amplify regions of IS3 insertion. Amplicon sizes were determined by gel electrophoresis and bands excised for sequencing. A sucrose-sensitive scrB knockout mutant of UA159 was constructed and selection applied for resistance to sucrose. Results: None of 10 strains examined carried all five IS elements at the same locations as UA159. Four carried IS3A, IS3C, IS3D and/or IS3F in the same location as UA159 and gave PCR amplicons of the same size. None of the strains showed the same amplicons as UA159 for IS3B or IS3E. In the case of IS3A, IS3C or IS3F, strains yielded either a 3 kb amplicon identical to that from UA159 and consistent with the presence of the IS element or an amplicon of 0.4 kb predicted if there had been no insertion. Insertion was concluded to have been by means of recombination of short direct sequence repeats, identified by sequencing. 16 mutants obtained by positive selection for sucrose-resistant colonies of a scrB mutant of UA159 were screened by PCR. The majority were concluded to be due to point mutations or deletions, but one was found to be due to transposition of an IS3 element into the scrA gene. Conclusion: Strains of S. mutans show considerable variation in carriage and site of insertion of IS3 elements. Active transposition of intact IS3 elements can inactivate genes and hence be an agent of genetic change.