PHYSIOLOGICAL SIGNIFICANCE OF TYROSINE TRANSPORT-SYSTEMS IN HALOBACTERIUM-SALINARIUM

We have developed a method for the separate determination of the activ ity of three tyrosine transport systems in Halobacterium salinarium ce lls. Two systems are specific for tyrosine, and the third is common to all aromatic amino acids. We have shown that the specific transport s ystem with high affinity is regulated (K(M) = 1.5.10(-8) M) by the rep ression-derepression type, of mechanism and that aromatic amino acids, particularly tyrosine itself, serve as effectors. The regulation of t he activity of the high affinity system is achieved both by tyrosine i tself and by other aromatic amino acids present in the medium. The con centration of phenylalanine which completely inhibits the activity of this system, is 2.5-5 muM. The activity of the specific system depends only slightly on the age of the culture, and the decrease in the tran sport activity as regards tyrosine, observed as the culture becomes ol der, is mainly due to a decrease in the activity of the common transpo rt system. The physiological significance of the multiplicity of H. sa linarium transport systems is discussed.