REGULATION OF LEUCINE TRANSPORT BY INTRACELLULAR PH IN BACILLUS-PASTEURII

The kinetics, specificity and mechanism of leucine uptake were studied in the alkaliphilic bacterium Bacillus pasteurii DSM 33 (ATCC 11859). Leucine was accumulated up to 200-fold by a sodium-dependent secondar y transport system for branched-chain amino acids. Apparent K-t values of 9.6 mu M for leucine, 8.9 mu M for isoleucine, 9.3 mu M for valine , and 0.71 mM for sodium were determined, and maximum uptake activity was observed at an external pH of 8.5 and at 35 degrees C. The effect of several ionophores indicated that transport was energized by the me mbrane potential and a sodium gradient; each gradient alone was suffic ient to drive the uptake of leucine. The activity of the leucine trans port system was regulated by the intracellular pH and was inhibited at an internal pH below 7.0.