a Heterocyst-Specific ATP-Binding Cassette Transporter

The devBCA operon, encoding subunits of an ATP-binding cassette exporter, is essential for differentiation of N2-fixing heterocysts in Anabaena spp. Nitrogen deficiency-dependent transcription of the operon and the use of its transcriptional start point, located 762 (Anabaena variabilis strain ATCC 29413-FD) or 704 (Anabaena sp. strain PCC 7120) bp upstream of the translation start site, were found to require the global nitrogen transcriptional regulator NtcA. Furthermore, NtcA was shown to bind in vitro to the promoter of devBCA. Nitrogen-fixing cyanobacteria are phototrophic microorgan-isms that use the energy of sunlight to reduce CO2 and N2 from the air, utilizing water as the reductant. Oxygen derived from photooxidation of water is highly damaging to nitrogenase, the enzyme responsible for the reduction of molecular nitrogen. Some genera (e.g., Anabaena) of multicellular cyanobacteria respond to deprivation of combined nitrogen by differentiation of about every 10th cell of a filament into a heterocyst, a cell specialized for the task of N2 fixation (25). Developing hetero-cysts lose the capacity to fix CO2, enhance respiration, an