Ammonium and methylammonium transport by the nitrogen-fixing bacterium Azotobacter vinelandii

Abstract

Azotobacter vinelandii, grown with NH4' as nitrogen source, was shown to possess an active transport system which can take up NH4' against a concentration gradient of 58-fold. The properties of the NH4' uptake system were investigated with the NH4' analog CH3NH3'. The use of this analog was justified on the basis of the conclusion that the uptake of NH4+ and CH3NH3+ involves a common binding site, as shown by the competitive inhibition of CH3NH3+ uptake by NH4+ (Ki approximately 3 ,uM). A Lineweaver-Burk plot for CH3NH3+ uptake revealed a biphasic curve, suggesting the existence of two CH3NH3+ (NH4+) uptake systems with apparent Km's for CH3NH3+ equal to 61 ,M and 661 ,M. The uptake of CH3NH3+ was inhibited by arsenate, as well as by cyanide or carbonyl cyanide-mchlorophenyl hydrazone, indicating that phosphate bond energy is required. All free-living N2-fixing bacteria are known to respond to the presence of fixed nitrogen in the environment by shutting off synthesis of nitrogenase. The regulatory mechanism which re

Ammonium (methylammonium) transport by Azospirillum spp.

“…These data argue for the existence of a specific carrier for CH3NH3 ~ and NH~-. The findings that the affinity of the carrier is considerably higher for NH~-than for CH3NH + , and that CH3NH ~ is toxic for [12] and Rhodospirillum rubrum (submitted), while the transport system of Azotobacter vinelandii seems to be constitutive [13].…”

Bacterial ammonium transport

“…Since the size of the steady-state NH~" pool depends on the ratio of NH~ influx over NH 3 efflux plus metabolism, an additional NH~ import should increase the NH2 pool. Alternatively, even in the absence of a second carrier, this ratio might be higher for NH~ than for MA, if the natural substrate (NH~) is transported faster than the analog (MA), as shown for some strains [9,11,50]. As will be discussed in section 6.3., the back diffusion rate of both molecules is about equal.…”

Survey of ammonium (methylammonium) transport by aerobic N2-fixing bacteria—the special case of Rhizobium

“…NH + transport systems are now established to occur in some lower eukaryotes and bacteria [1][2][3][4][5][6][7]. Most of the bacterial species investigated can fix N 2.…”