Versatile Properties of a Nonsaturable, Homogeneous Transport System in Bacillus subtilis: Genetic, Kinetic, and Affinity Labeling Studies

The multiphasic kinetics that characterize the transport of many amino acids intoBacillus subtilis suggestsa priori at least two mechanisms: (i ) a heterogeneous mixture of two independent transport processes, or (ii ) a single, homogeneous system that might involve a negative cooperative mechanism. The highly specific transport system for L-tyrosine and L-phenylalanine inB. subtilis was studied as a case in point. The possible presence of a mixed system of independent transport systems was negated by the retention of multiphasic kinetics of transport in two types of permease mutants. Furthermore, evaluation of kinetic data obtained during transport under various uptake conditions of pH and temperature, or in the presence of metabolic inhibitors, did not reveal the heterogeneity expected of mechanism (i ). These data, taken together with characteristics of substrate specificity and affinity labeling, provide substantial support for a negative cooperative mechanism for L-tyrosine and L-phenylalanine transport.