L-(-LACTATE DEHYDROGENASE-DEFICIENCY IS LETHAL IN STREPTOCOCCUS-MUTANS())

The previously cloned gene for L-(+)-lactate dehydrogenase (LDH) from Streptococcus mutans was mutagenized in vitro. An Escherichia coli tra nsformant which expressed a thermolabile LDH activity was identified. The Idh(Ts) gene was introduced into S. mutans on a suicide vector to create a heterodiploid expressing both wild-type and thermolabile LDH activities. Self-recombinants which had only one Idh gene were isolate d. One of these clones expressed only the thermolabile LDH activity. T his isolate grew well at 30 degrees C but did not grow at 42 degrees C under a variety of cultivation conditions, thereby proving that LDH d eficiency is lethal in S. mutans in the absence of compensatory mutati ons.