TABLE 1.

Substrate specificity of QacT of L. plantarum ATCC 14917^a

Substrate analogue	Rate of glycine betaine uptake (%)	Rate of proline uptake (%)
4-Aminobutyric acid	115	55
Dimethylglyine	90	<5
Trimethylamine	105	<5
Tetramethylammonium	35	<5
Proline	100	<5
Dimethylsulfonium propionic acid	30	<5
Carnitine	<5	<5
Glycine betaine	<5	<5

^aThe cells were cultured on CDM plus 0.8 M KCl and subsequently washed and resuspended in 50 mM potassium phosphate (pH 6.5) to a final protein concentration of 0.22 mg/ml. Prior to the transport reactions, the cells were incubated for 5 min at 30°C with 10 mM glucose. Uptake was initiated by adding [¹⁴C]glycine betaine or [¹⁴C]proline (final concentration, 1.25 mM) plus a 100-fold excess of substrate analogue. An uptake activity of 100% corresponds to rates of 33 and 17 nmol/min per mg of protein for glycine betaine and proline, respectively.