Structure and transcriptional regulation of the gene encoding pyruvate formate-lyase of a ruminal bacterium, Streptococcus bovis

The gene (pfl) encoding pyruvate formate-lyase (Pfl) from Streptococcus bovis was sequenced. The deduced amino acid sequence of Pfl was similar to Streptococcus mutans Pfl, and included the conserved regions necessary for free-radical formation and a catalytic site. The Pfl of S. bovis appeared to be a free-radical-containing enzyme because of its dioxygen sensitivity and its amino acid sequence similarity with the Escherichia coli enzyme. The pfl mRNA of S. bovis was approximately 2.3 kb and was transcribed in a monocistronic fashion. When cells were grown in batch culture at ph 6.9, the level of pfl transcript increased as the growth phase changed from exponential growth to stationary phase. This result was in constrast to the previous observation that the level of lactate dehydrogenase (Ldh) mRNA decreased during the later stages of growth. Continuous culture experiments conducted at ph 6.9 under glucose-limited and ammonia-limited conditions revealed that pfl mRNA was decreased by an excess supply of glucose, as well as by a high growth rate. On the contrary, Idh mRNA increased when excess glucose was supplied and the growth rate was high. The amount of pfl mRNA in cells was lower at ph 4.5 than ph 6.9, whereas the level of Idh mRNA was higher at ph 4.5. This result was consistent with the amounts of Pfl and Ldh in cells and the proportion of formate and lactate produced. These results support the hypothesis that S. bovis regulates Pfl and Ldh synthesis at the transcriptional level in response to growth conditions.