Glutamine and glutamate transport by Anabaena variabilis

Abstract

Anabaena variabilis, a dinitrogen-fixing cyanobacterium, has high- and low-affinity systems for the transport of glutamine and glutamate. The high-affinity systems have K(m) values of 13.8 and 100 μM and maximal rates of 13.2 and 14.4 nmol·min-1·mg of chlorophyll α-1 for glutamine and glutamate, respectively. The low-affinity systems have K(m) values of 1.1 and 1.4 mM and maximal rates of 125 and 100 nmol·min-1·mg of chlorophyll a-1 for glutamine and glutamate, respectively. Glutamine was unable to support growth of A. variabilis in the absence of any other nitrogen source, and glutamate alone at 500 μM was inhibitory to its growth. The analog L-methionine-DL-sulfoximine (MSX) was transported by a high-affinity system with a K(m) of 34 μM. Competition experiments and the transport characteristics of a specific class of MSX-resistant mutants imply that glutamine, glutamate, and MSX share a common component for transport. A second class of MSX-resistant mutants had a glutamine synthetase activity with altered affinity constants for glutamine and glutamate relative to the wild-type enzyme.