Restricted cellular differentiation in cyanobacterial filaments

Abstract

Although commonly considered simple organisms that grow as single cells, many bacteria can grow as filaments made of chains of cells. In cyanobacteria, prokaryotes that perform oxygen-evolving photosynthesis, filamentous forms developed early in evolution (1). In the most complex filamentous cyanobacteria, as many as four cell types can be found: vegetative cells that perform oxygenic photosynthesis and concomitant CO2 fixation; the cells of hormogonia, which are small motile filaments frequently made of small cells; akinetes, which are resting cells; and heterocysts, which are terminally differentiated cells specialized in the fixation of atmospheric nitrogen (2, 3). In a cyanobacterial filament, the heterocysts differentiate from some vegetative cells in response to nitrogen deficiency (2⇓–4). In cyanobacteria of the genera Anabaena or Nostoc , heterocysts form intercalary along the filament, producing a nonrandom pattern of one heterocyst to approximately 10 to 15 vegetative cells. However, can any cell in a filament differentiate into a heterocyst? In PNAS, Risser et al. (5) present evidence consistent with the idea that, at a given time, only some cells in the filament have the capability to differentiate. Nitrogenase, the enzyme that catalyzes N2 fixation producing ammonia, is oxygen-labile, but the heterocysts provide a microoxic environment for its function (2⇓–4). The process of heterocyst differentiation involves the execution of a specific program of gene expression that includes the induction of regulatory genes, some of which act early in the process, and of genes encoding the proteins for the morphological and biochemical differentiation of the heterocyst, including nitrogenase (6). When a source of combined nitrogen is exhausted from the culture medium, the cells in the filament sense nitrogen … [↵][1]1E-mail: eflores{at}ibvf.csic.es. [1]: #xref-corresp-1-1