The Major Autolysin of Streptococcus gordonii Is Subject to Complex Regulation and Modulates Stress Tolerance, Biofilm Formation, and Extracellular-DNA Release

A gene, designated atlS, encoding a major autolysin from Streptococcus gordonii, was identified and characterized. The predicted AtlS protein is 1,160 amino acids and 127 kDa and has a conserved beta 1,4-N-acetylmuramidase domain. Zymographic analysis of wild-type S. gordonii revealed peptidoglycan hydrolase activities with molecular masses of 130 and 90 kDa that were absent in an atlS deletion mutant. Western blotting revealed that the 90-kDa band was derived from the 130-kDa protein. Inactivation of atlS resulted in formation of long chains by the cells, markedly decreased autolytic capacity, poor biofilm formation, diminished tolerance of acid and oxidative stress, and decreased production of extracellular DNA (eDNA). The biofilm-forming capacity of the atlS mutant could be almost completely restored to that of the wild-type strain by adding purified recombinant AtlA autolysin of S. mutans but was only partially restored by addition of eDNA. Autolysis, eDNA release, and atlS expression increased sharply when cells entered stationary phase and were greatly enhanced in cells growing with aeration. The LytST and VicRK two-component systems were both required for the induction of atlS by aeration, and purified LytT was able to bind to the promoter region of atlS in vitro. Thus, AtlS and its associated regulatory cascade dominantly control phenotypes of S. gordonii that are critical to colonization, persistence, and competition with other commensal and pathogenic oral bacteria in response to the redox environment and growth domain.