Recently it has been shown that Streptococcus thermophilus is competent for natural genetic transformation. This property is widespread among streptococci and may include all members of the genus. Upon entering the competent state, streptococci start transcribing a number of competence-specific genes whose products are required for binding, uptake and processing of transforming DNA. In addition to the core competence genes, competent streptococci express a number of accessory genes that are dispensable for transformation in the laboratory, but presumably play an important role under natural conditions. In Streptococcus pneumoniae, one of these accessory genes encodes a competence-specific murein hydrolase termed CbpD. Experimental evidence indicates that pneumococcal CbpD is part of a predatory mechanism that lyses noncompetent sister cells or members of closely related species in order to release homologous DNA that can be taken up by the competent attacker cells. Competent S. thermophilus LMG18311 cells produce a CbpD-like protein, Stu0039, which might have the same or a similar function. In the present study we have characterized this protein and shown that it is a murein hydrolase with a novel type of cell surface-binding domain. Furthermore, we show that Stu0039 is rapidly inactivated by H(2)O(2) produced during aerobic growth of S. thermophilus. We propose that this inactivation mechanism has evolved for self-protection purposes to prevent extensive autolysis in a competent population. Interestingly, in contrast to pneumococcal CbpD, which does not affect the transformation properties of the producer strain, deletion of Stu0039 reduces the transformability of S. thermophilus.