Regulation of lactate dehydrogenase synthesis in a ruminal bacterium, Streptococcus bovis.

Lactate dehydrogenase (LDH) was purified from three strains of Streptococcus bovis, and the gene ldh was cloned and sequenced. The ldh of S. bovis from a goat (TH1) was different from the ldhs of two other strains from cattle (TH2, JB1) in that Asp(220) was substituted for Glu. Northern blot analysis revealed that the LDH-mRNA of S. bovis was approximately 1.0 kbp, which was transcribed in a monocistronic fashion. When cells were grown at pH 6.9 in a batch culture, the level of ldh transcript decreased as the growth phase changed; from exponential growth to the cessation of growth. The level of ldh transcript was higher in cells grown at pH 4.5 than at pH 6.9. This observation was consistent with the amounts of LDH in cells and the percentages of lactate produced. These results support the hypothesis that S. bovis regulates LDH synthesis at the transcriptional level probably in response to intracellular pH.