AMMONIUM METHYLAMMONIUM PERMEASES OF A CYANOBACTERIUM - IDENTIFICATION AND ANALYSIS OF 3 NITROGEN-REGULATED AMT GENES IN SYNECHOCYSTIS SP. PCC-6803/

Ammonium is an important nitrogen source for many microorganisms and p lants. Ammonium transporters whose activity can be probed with [C-14]m ethylammonium have been described in several organisms including some cyanobacteria, and amt genes encoding ammonium/methylammonium permease s have been recently identified in yeast, Arabidopsis thaliana, and so me bacteria. The unicellular cyanobacterium Synechocystis sp. PCC 6803 exhibited a [C-14]methylammonium uptake activity that was inhibited b y externally added ammonium. Three putative amt genes that are found i n the recently published complete sequence of the chromosome of strain PCC 6803 were inactivated by insertion of antibiotic resistance-encod ing gene-cassettes. The corresponding mutant strains were impaired in uptake of [C-14]methylammonium. Open reading frame sll0108 (amt1) was responsible for a high affinity uptake activity (K-s for methylammoniu m, 2.7 mu M), whereas open reading frames sll11017 (amt2) and sll0537 (amt3) made minor contributions to uptake at low substrate concentrati ons. 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 structure of the cyanobacterial prom oters activated by the nitrogen control transcription factor, NtcA.