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.

The efficiency of conjugal transfer of plasmids from Escherichia coli to the cyanobacterium Anabaena sp. strain PCC 7120 was quantitated as a function of the number of restriction sites for the restriction enzymes carried by the recipient. In addition to the previously recognized isoschizomers of AvaI and AvaII, PCC 7120 was found to possess an isoschizomer of AvaIII. Plasmids modified in E. coli with methylases that protect in vitro against restriction by the three enzymes were transferred with high efficiency, nearly independent of the number of restriction sites on the plasmid. Plasmids left unprotected against one of the three restriction enzymes were transferred with lower efficiencies. For low numbers of sites, the efficiency of conjugal transfer decreased as an exponential function of the number of unprotected sites. The methods presented may be used to increase the efficiency of conjugal transfer into restriction-competent bacteria.

Nitrogen sources commonly used by cyanobacteria include ammonium, nitrate, nitrite, urea and atmospheric N₂, and some cyanobacteria can also assimilate arginine or glutamine. ABC (ATP-binding cassette)-type permeases are involved in the uptake of nitrate/nitrite, urea and most amino acids, whereas secondary transporters take up ammonium and, in some strains, nitrate/nitrite. In cyanobacteria, nitrate and nitrite reductases are ferredoxin-dependent enzymes, arginine is catabolized by a combination of the urea cycle and arginase pathway, and urea is degraded by a Ni²⁺-dependent urease. These pathways provide ammonium that is incorporated into carbon skeletons through the glutamine synthetase–glutamate synthase cycle, in which 2-oxoglutarate is the final nitrogen acceptor. The expression of many nitrogen assimilation genes is subjected to regulation being activated by the nitrogen-control transcription factor NtcA, which is autoregulatory and whose activity appears to be influenced by 2-oxoglutarate and the signal transduction protein P_II. In some filamentous cyanobacteria, N₂ fixation takes place in specialized cells called heterocysts that differentiate from vegetative cells in a process strictly controlled by NtcA.

Cyanobacteria carry out oxygenic photosynthesis, play a key role in the cycling of carbon and nitrogen in the biosphere, and have had a large impact on the evolution of life and the Earth itself. Many cyanobacterial strains exhibit a multicellular lifestyle, growing as filaments that can be hundreds of cells long and endowed with intercellular communication. Furthermore, under depletion of combined nitrogen, filament growth requires the activity of two interdependent cell types: vegetative cells that fix CO2 and heterocysts that fix N2. Intercellular molecular transfer is essential for signaling involved in the regulation of heterocyst differentiation and for reciprocal nutrition of heterocysts and vegetative cells. Here we review various aspects of multicellularity in cyanobacterial filaments and their differentiation, including filament architecture with emphasis on the structures used for intercellular communication; we survey theoretical models that have been put forward to understand heterocyst patterning and discuss the factors that need to be considered for these models to reflect the biological entity; and finally, since cell division in filamentous cyanobacteria has the peculiarity of producing linked instead of independent cells, we review distinct aspects of cell division in these organisms.