A Mutant of Streptococcus j&em&s Defective in Phosphate Uptake*

Résumé fourni par la source

SUMMARY Uptake of phosphate by Streptococcus faecalis ATCC 9790 was strictly dependent upon metabolic energy and was stimu- lated by K+. Glycolysis supported rapid uptake. Cells adapted to growth on arginine could couple slow phosphate uptake to arginine fermentation. Uptake of “Pi was primarily net uptake rather than exchange wit.h the intracellular phosphorus pool. “Pi entering the cells was rapidly incorporated into glycolytic intermediates. By means of a 32P “suicide” method, a mutant was isolated which required elevated levels of Pi for growth at pH 7, and did found to be defective in phosphate uptake; glycolysis, arginine fermentation, K+ uptake, and 32P efflux were the same in mutant as in the parent strain. The rate of phosphate uptake in both strains followed satura- tion kinetics. In strain 9790 the apparent dissociation constant was about 10-5 M and was unaffected by pH from 5 to 8. By contrast, in PT-1 the dissociation constant increased rapidly with pH. The rate of phosphate uptake in strain 9790 was thus relatively independent of pH, while in PT-1 the rate was optimal at pH 6 and then declined, becoming negligible above 7.5. When arginine was used as energy source, the pH dependence of phosphate uptake by strains 9790 and PT-1 was the same as on glucose. At pH 6, strain 9790 had a much higher affinity for Pi than did mutant PT-1. The defect in phosphate upt.ake by PT-1 thus appears to be independent of the nature energy source. The characteristics of phosphate uptake by protoplasts of strains 9790 and PT-1 were the same as those of intact cells. Removal of the cell wall affected neither the pH dependence nor the apparent dissociation constants of uptake at pH 6. Strains 9790 and PT-1 were compared with respect to glyceral- dehyde phosphate dehydrogenase. In both strains virtually all the enzyme was found in supernatant after metabolic lysis and no qualitative differences were found between enzyme prep- arations from the two strains. The results do not support hypothesis that phosphate uptake results from interaction of glyceraldehyde phosphate dehydrogenase with extracellular phosphate. It is proposed that mut.ant PT-1 is defective in an access system which mediates the entry of phosphate into S. faecah. Acknowledgments-We are grateful to Susan Sylvan for expert technical assistance, and to Irene Abelow (Department of Microbiology, University of Colorado School of Medicine) for the antigenic classification of our bacterial strains.

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