Synthesis of nitrogenase and heterocysts by Anabaena sp. CA in the presence of high levels of ammonia

Abstract

Anabaena sp. CA fails to synthesize heterocysts and nitrogenase when grown with KNO3 as the nitrogen source. By contrast, both heterocysts and proheterocysts are synthesized in NH4Cl-containing media to a level nearly commensurate with cells grown in the absence of combined nitrogen. The growth rate of the organism in NH4Cl-containing media was similar to that obtained with KNO3 as the N source and was independent of the presence of N2 in the atmosphere. The organism apparently assimilated nitrate and ammonium N equally well to meet the N requirements for growth. Moreover, in contrast to previous studies with other cyanobacteria, the repressor signal for heterocyst differentiation in Anabaena sp. CA is not derived from the metabolism of NH3 but appears to be involved with nitrate metabolism. Nitrogenase activity was partially expressed in NH4Cl-grown cultures. Increasing the level of nitrogenase activity to a value representative of a N2-grown culture required both the inhibition of NH3 assimilation and de novo protein synthesis. An increase in the number of mature heterocysts was not required. The fact that high levels of exogenous NH3 only partially repress the synthesis of protein required for the maximum expression of nitrogenase activity in Anabaena sp. CA has important implications.