Genetic Variation of the gbpA Region in Streptococcus mutans

Objectives: Streptococcus mutans is the major aetiological agent of dental caries in humans, and glucosyltransferase-mediated synthesis of glucan polymers from sucrose, aided by glucan-binding proteins (Gbps), contributes to colonisation of the tooth surface. The apparent contribution of Gbps to the virulence of S. mutans necessitates further investigation of these proteins. The acquisition of the complete genome sequence of S. mutans UA159 has enabled direct comparison with the sequence of selected features of other S. mutans strains. The purpose of this study was to determine the genetic variation previously found in the gene encoding GbpA in S. mutans, using strain V1996 as an example.

Methods: Primers were designed based on the sequence within and surrounding the gbpA gene of S. mutans UA159, and used to amplify DNA from strains UA159 and V1996. Western blots were carried out with specific GbpA antiserum and biotin-labelled dextran.

Results: Primers internal to the gbpA gene yielded a PCR product for S. mutans UA159 but not V1996. External primers flanking gbpA generated a product of 3 kb in UA159 but a 10 kb product in V1996. Sequencing of this 10 kb fragment determined the end-points of the insert and identified sequence not present in UA159. Western blotting showed that V1996 did not express GbpA.

Conclusions: These experiments confirmed the absence of gbpA and insertion of a sequence of DNA that is novel to S. mutans strain V1996, which results in non-expression of GbpA and may introduce new functions.