Epistatic Relationships of Two Regulatory Factors During Heterocyst Development

Abstract

Cyanobacteria, previously known as blue-green algae, are a large and diverse group of photosynthetic bacteria representing one of the major phylogenetic lines of eubacteria (Carr and Whitton 1982). Being widely distributed in nature in terrestrial, freshwater, and marine habitats, cyanobacteria are also often dominant photosynthetic organisms in extreme environments, including hot springs, saline lakes, and desert soils. Blooms of cyanobacteria may develop in fresh water lakes in times when they are rich in nutrients. Some filamentous cyanobacteria exhibit an interesting example of cellular differentiation to produce heterocysts, highly specialized cells that fix atmospheric nitrogen. The differentiation of a photosynthetic vegetative cell into a nitrogen-fixing heterocyst is regulated by the availability of combined nitrogen and intercellular communication, and results in a semi-regular pattern of heterocysts along the vegetative cell filament. Anabaena sp. strain PCC 7120 (Anabaena PCC 7120) grows as filaments of vegetative cells in media containing combined nitrogen. Vegetative cells carry out oxygenic photosynthesis similar to that of algae and higher plants (Ho and Krogmann 1982). When Anabaena PCC 7120 is grown in the absence of combined nitrogen, one out of approximately ten vegetative cells differentiates into a heterocyst. The terminally differentiated heterocysts are highly specialized cells in which nitrogen fixation occurs (Fay 1992). The oxygen-sensitive nitrogenase, the main enzyme complex for catalyzing the reduction of dinitrogen to ammonia, is present only in heterocysts (Bergman et al. 1986). The thick and non-permeable envelope of the heterocyst provides anaerobic conditions for nitrogenase in aerobically growing filaments (Walsby 1985). The differentiation of a vegetative cell into a heterocyst takes approximately 24 hours and culminates with the expression of the nif genes and the reduction of nitrogen to ammonia. Heterocyst differentiation involves a Algae Volume 24(2): 85-91, 2009