Diversity of surface protein antigen A gene in the chromosomal DNA of Streptococcus sobrinus

A recombinant plasmid, pYA724, containing an 8.45-kb DNA fragment encoding surface protein antigen A (spaA) from Streptococcus sobrinus 6715 was used to examine the DNA homology of the spaA gene with chromosomal DNA of Streptococcus sobrinus B13N. Restriction endonuclease BamHI-digested pYA724 DNA was 32P radio-labeled by nick-translation, and a Southern blot DNA-DNA hybridization experiment was carried out. pYA724 DNA hybridized with a BamHI fragment of B13N chromosomal DNA, and the size of this fragment was larger than 8.45 kb. The chromosomal DNA was digested with BamHI-Pst I, BamHI-Hind III, and BamHI-Sal I, and analyzed by Southern blot DNA hybridization. pYA724 DNA hybridized with different sizes and numbers of DNA fragments of B13N chromosomal DNA; however, the sizes were not identical to those of internal fragments expected from the pYA724 restriction map. SpaA gene clone derivative plasmids, pYA745 and pYA756, were digested with EcoRI and Hinc II, respectively, and electrophoresed together with the digested B13N chromosomal DNA on the same gel; then Southern blot analysis was carried out. Fragments with sizes corresponding to the EcoRI or HincII internal fragments of pYA745 and pYA756 were not observed among the B13N fragments. These data indicate that serotype d S. sobrinus B13N has DNA homologous with the spaA gene from serotype g strain 6715, although within the same biotype strain has a diversity of arrangement within the chromosome.