Low-affinity, high-capacity system of glucose transport in the ruminal bacterium Streptococcus bovis: Evidence for a mechanism of facilitated diffusion

Abstract

The glucose phosphotransferase system (PTS) of Streptococcus bovis could not account for the glucose consumption of exponential cultures, and the kinetics of glucose transport were biphasic. A PTS-deficient mutant lost the high-affinity, low-capacity system but retained its ability to take up glucose at high substrate concentrations. The low-affinity, high-capacity system did not require a proton motive force or ATP and could not be driven by an artificial membrane potential in the presence or absence of sodium. Since low-affinity transport was directly proportional to the external substrate concentration and exhibited counterflow kinetics, it appeared that a facilitated-diffusion mechanism was responsible for glucose transport at high substrate concentrations.