Involvement of phosphoenolpyruvate in lactose utilization by group N streptococci.

The effect of sodium fluoride on lactose metabolism and o-nitrophenyl-beta-d-galactopyranoside (ONPG) hydrolysis by Streptococcus lactis strains 7962 and C(2)F suggested that different mechanisms of lactose utilization existed in the two strains. Sodium fluoride prevented lactose utilization and ONPG hydrolysis by whole cells of S. lactis C(2)F but had no effect on S. lactis 7962. Although hydrolysis of ONPG by toluene-treated cells of S. lactis 7962 occurred without addition of phospho-enolpyruvate (PEP), toluene-treated cells of S. lactis C(2)F required the presence of this cofactor. Concentrated cell extracts of S. lactis C(2)F hydrolyzed ONPG; this hydrolysis was inhibited by NaF, but the addition of PEP, in the presence of NaF, restored maximal activity. Addition of acetyl-phosphate, carbamyl-phosphate, adenosine-5'-triphosphate, guanosine-5'-triphosphate, or uridine-5'-triphosphate did not stimulate activity. The presence of cofactors did not stimulate and NaF did not inhibit the hydrolysis in extracts of S. lactis 7962. To confirm the operation of two mechanisms, S. lactis 7962 was shown to hydrolyze lactose to glucose and galactose, whereas S. lactis C(2)F was unable to split the disaccharide. In addition, whole cells of S. lactis C(2)F rapidly accumulated a phosphorylated derivative of thiomethyl-beta-d-galactoside (TMG) which behaved chromatographically and electrophoretically like TMG-PO(4). Unexpectedly, S. lactis 7962 also accumulated a TMG derivative, although the rate was extremely low. These data indicate that different mechanisms of lactose utilization exist in the two strains, with a phosphorylation step dependent on PEP involved in S. lactis C(2)F.