Presence of NAD+-specific glyceraldehyde-3-phosphate dehydrogenase and CcpA-dependent transcription of its gene in the ruminal bacterium Streptococcus bovis.

To know the role of NADP+-specific glyceraldehyde-3-phosphate dehydrogenase (GAPN) in Streptococcus bovis, the molecular properties and transcriptional control of the gene encoding GAPN (gapN) were examined. The GAPN in S. bovis was deduced to consist of 476 amino acids with a molecular mass of 51.1 kDa. The gapN gene was transcribed in a monocistronic fashion. GAPN synthesis appeared to be regulated at the transcriptional level in response to changes in growth conditions. In a mutant that lacks the ccpA gene encoding catabolite control protein A (CcpA), the gapN-mRNA level was lower than in the parent strain. A binding site of CcpA was found in the upper region of gapN. These results suggest that transcription of gapN is regulated through CcpA. Overexpression of GAPN in S. bovis did not affect the growth rate or formate-to-lactate ratio, suggesting that the flux in the glycolytic pathway is unlikely to be altered by GAPN activity. Streptococcus bovis GAPN was NADP+ dependent, but not phosphate dependent. In addition, S. bovis did not have other NADPH-producing systems such as the hexose monophosphate pathway and NADPH:NAD+ oxidoreductase. Therefore, GAPN may play an important role in NADPH production in S. bovis.