FIGURE 3. Relative transport % of l-[¹⁴C]cystine uptake in S. mutans UA159 in the presence of a 100-fold excess of non-radioactive compounds.

The specificity of tcyABC-mediated amino acid uptake was examined using an amino acid competition assay. Briefly, the uptake of l-[¹⁴C]cystine (4 µM) was measured in the presence of 400 µM of the following cold l-amino acids: arginine, cysteine, glutamine, glutamate, leucine, and methionine. As a positive control and to determine total l-[¹⁴C]cystine uptake, cells were incubated with radio-labeled cystine with no competing substrate. As a negative control, a reaction containing no cells was incubated with l-[¹⁴C]cystine and no radioactivity was detected. Rate of uptake was calculated using three independent experiments, each subjected to duplicate sampling procedures. Uptake was normalized using dry cell weights and final uptake was calculated as nmol of substrate per mg of cells per minute.