Molecular characterization and significance of phosphoenolpyruvate carboxykinase in a ruminal bacterium, Streptococcus bovis.

To gain knowledge about the significance of phosphoenolpyruvate (PEP) carboxykinase (PCK) in Streptococcus bovis, the sequence of the gene encoding PCK (pck) was determined. Transcriptional analysis indicated that the pck is transcribed in a monocistronic fashion. The level of pck-mRNA was higher when cells were grown on lactose than on glucose, suggesting that PCK synthesis increases when the growth rate is low. The pck-mRNA level was higher in a mutant lacking ccpA, which encodes the catabolite control protein A (CcpA), than in the parent strain, suggesting that pck transcription is suppressed by CcpA. S. bovis PCK showed oxaloacetate (OAA)-decarboxylating activity, but no PEP-carboxylating activity (reverse reaction). In S. bovis, OAA was speculated to be produced from PEP via pyruvate. Disruption of pck in S. bovis resulted in decreased growth rate and cell yield. When a pck-disrupted mutant was grown in a medium lacking amino acids, the lag phase was longer and the cell yield was lower than the case of the parent strain. These results suggest that pck is involved in the initiation of growth, including the induction of amino acid synthesis and energy metabolism.