Heterocyst development and localization of cyanophycin in N-2-fixing cultures of Anabaena sp PCC 7120 (cyanobacteria)

The process of N-2 fixation in the filamentous cyanobacterium Anabaena sp. PCC 7120 is known to occur in terminally differentiated cells called hetero cysts. This study is concerned with a morphological and immunocytochemical analysis of the developing heterocysts. The heterocysts continue a developm ental process after synthesis of the specialized cell wall and the formatio n of the proheterocyst. The initial stages were described by Wilcox et al. (1973) and designated stages 1 through 7, with stages 5-7 associated with t he maturing heterocyst. We now designate a stage 8 as the postmaturation st age, based on physiological and ultrastructural evidence. Immunocytochemist ry to detect the nitrogenase protein NifH and the nonribosomally synthesize d polypeptide cyanophycin demonstrated a complementary accumulation of thes e polypeptides. Accumulation of the nitrogenase protein was greatest at sta ges 5 and 6 and then declined precipitously. Cyanophycin was more prevalent after late stage 6 and was primarily associated with the polar nodule (pol ar plug) and the neck connecting the heterocyst with the adjoining vegetati ve cell. We suggest that the cyanophycin-containing; polar plug is a key in termediate in the storage of fixed nitrogen in the heterocyst, a result con sistent with the suggestion first made by Carr (1988) that cyanophycin exis ts as a dynamic reservoir of fixed nitrogen within the heterocysts.