Nitrogen Control in Cyanobacteria

Abstract

Nitrogen is a quantitatively important bioelement which is incorporated into the biosphere through assimilatory processes carried out by microorganisms and plants. Numerous nitrogen- containing compounds can be used by different organisms as sources of nitrogen. These include, for instance, inorganic ions like nitrate or ammonium and simple organic compounds like urea, amino acids, and some nitrogen-containing bases. Addi- tionally, many bacteria are capable of fixing N2. Nitrogen con- trol is a phenomenon that occurs widely among microorgan- isms and consists of repression of the pathways of assimilation of some nitrogen sources when some other, more easily assim- ilated source of nitrogen is available to the cells. Ammonium is the preferred nitrogen source for most bacteria, but glutamine is also a very good source of nitrogen for many microorgan- isms. Two thoroughly investigated nitrogen control systems are the NtrB-NtrC two-component regulatory system found in en- terics and some other proteobacteria (80) and the GATA family global nitrogen control transcription factors of yeast and some fungi (75). Novel nitrogen control systems have, how- ever, been identified in bacteria other than the proteobacteria, like Bacillus subtilis (26), Corynebacterium glutamicum (52), and the cyanobacteria. The cyanobacterial system is the subject of this review. The cyanobacteria are prokaryotes that belong to the Bac- teria domain and are characterized by the ability to perform oxygenic photosynthesis. Cyanobacteria have a wide ecological distribution, and they occupy a range of habitats, which in- cludes vast oceanic areas, temperate soils, and freshwater lakes, and even extreme habitats like arid deserts, frigid lakes, or hot springs. Photoautotrophy, fixing CO2 through the Calvin cycle, is the dominant mode of growth of these organisms (109). A salient feature of the intermediary metabolism of cyanobacteria is their lack of 2-oxoglutarate dehydrogenase (109). As a consequence, they use 2-oxoglutarate mainly as a substrate for the incorporation of nitrogen, a metabolic ar- rangement that may have regulatory consequences. Notwith- standing their rather homogeneous metabolism, cyanobacteria exhibit remarkable morphological diversity, being found as either unicellular or filamentous forms and exhibiting a num- ber of cell differentiation processes, some of which take place in response to defined environmental cues, as is the case for the differentiation of N2-fixing heterocysts (109). Nitrogen control in cyanobacteria is mediated by NtcA, a transcriptional regulator which belongs to the CAP (the catab- olite gene activator or cyclic AMP (cAMP) receptor protein) family and is therefore different from the well-characterized Ntr system. Interestingly, however, the signal transduction PII protein, which plays a key role in Ntr regulation, is found in cyanobacteria but with characteristics which differentiate it from proteobacterial PII. In the following paragraphs, we shall first briefly summarize our current knowledge of the cyanobac- terial nitrogen assimilation pathways and of what is known about their regulation at the protein level. This description will introduce most of the known cyanobacterial nitrogen assimi- lation genes. We shall then describe the ntcA gene and the NtcA protein themselves to finally discuss NtcA function through a survey of the NtcA-regulated genes which partici- pate in simple nitrogen assimilation pathways or in heterocyst differentiation and function.