Induction of an AP endonuclease activity in Streptococcus mutans during growth at low pH

The oral microbe Streptococcus mutans uses adaptive mechanisms to withstand the fluctuating pH levels in its natural environment. The regulation of pr otein synthesis is part of the mechanism of acid adaptation and tolerance i n S. mutans. Here, we demonstrate that the organism's acid-inducible protei n repertoire includes an AP endonuclease activity. This abasic site-specifi c endonuclease activity is present at greater levels in cells grown at low pH than in cells grown at pH7, and is apparently independent of the RecA pr otein. Experiments using tetrahydrofuran or alpha-deoxyadenosine-containing substrates indicate that the activity induced at low pH may be similar to the activity of exonuclease III from E. coli. Acid-adapted S. mutans also s hows an increased survival rate after exposure to near-UV radiation in both the wild type and a recA strain. Far-UV radiation resistance is observed i n the wild type only. The endonuclease activity was purified approximate to 500-fold from an S. mutans recA mutant strain grown at pH 5. Initial chara cterization revealed a 3' to 5' exonuclease activity, and showed additional functional similarities to DNA repair enzymes from other organisms.