Ammonium/Methylammonium Permeases of a Cyanobacterium

Abstract

Ammonium is an important nitrogen source for many microorganisms and plants. Ammonium transporters whose activity can be probed with [14C]methylammo- nium have been described in several organisms includ- ing some cyanobacteria, and amt genes encoding ammo- nium/methylammonium permeases have been recently identified in yeast, Arabidopsis thaliana, and some bac- teria. The unicellular cyanobacterium Synechocystis sp. PCC 6803 exhibited a [14C]methylammonium uptake ac- tivity that was inhibited by externally added ammo- nium. Three putative amt genes that are found in the recently published complete sequence of the chromo- some of strain PCC 6803 were inactivated by insertion of antibiotic resistance-encoding gene-cassettes. The cor- responding mutant strains were impaired in uptake of [14C]methylammonium. Open reading frame sll0108 (amt1) was responsible for a high affinity uptake activ- ity (Ks for methylammonium, 2.7 ?M), whereas open reading frames sll1017 (amt2) and sll0537 (amt3) made minor contributions to uptake at low substrate concen- trations. Expression of the three amt genes was higher in nitrogen-starved cells than in cells incubated in the presence of a source of nitrogen (either ammonium or nitrate), but amt1 was expressed at higher levels than the other two amt genes. Transcription of amt1 was found to take place from a promoter bearing the struc- ture of the cyanobacterial promoters activated by the nitrogen control transcription factor, NtcA. Ammonium