Characteristics and Regulation of the Ammonium Transport System in Filamentous Nonheterocystous Cyanobacterium Plectonema boryanum

Abstract

Ammonium transport in Pectonema boryanum, a filamentous noneterocystous cyanoacterium was investigated in cells exposed to N-sufficient (medium with nitrate) or N-stress (medium without nitrate) conditions. T N-sufficient cells exhibited a single transport system (Km = 20 μM; Vmax = 258 mmol kg-1 protein min-1) and a 40-fold concentration gradient was formed following 10 min of transport. The transport was maximally operative at pH 7.0 and 33°C. The transport was not inhibited by methylamine (Km= 33.3 μM; Vmax=50 mmol kg-1 protein min-1). Following transfer o N-sufficient cells to N-stress conditions, the intracellular ammonium pool declined from 0. 38 to 0. 16 mM during the initial value during the first 6 h, but resumed to the initial value during the next 48 h. The N-stressed cells exhibited two ammonium transport systems with 〈high〉 and ⎒low affinity〉 respectively. The kinetics of the former system was similar to the N-sufficient cells, while the latter system had altered kinetics (Km = 50 μM; Vmax = 650 mmol kg-1 protein min-1). The results suggest that P. boryanum possesses multiple transport systems, and that their expression is regulated, depending upon the presence or absence of nitrate in the growth medium. © 1994, Gustav Fischer Verlag, Stuttgart. All rights reserved.