The regulation of nitrogenase derepression, plus the catalytic activity and protein concentration of glutamine synthetase (GS), nitrate reductase (NR), ribulose-l$-bisphosphate carbxylase/oxygenase (Rubisco) and phycoerythrin (PE) were studied in the filamentous non-heterocystous cyanobacterium PIectonemu boryanum PCC 73110. Both nitrogen limitation and microaerobic incubation were essential for the derepression of nitrogenase. Oxygen caused irreversible inactivation of nitrogenase, as well as repression of its synthesis. A temporal separation of N2 fixation and net photosynthetic O2 evolution was observed under a N2/C02 ( 9 5 5 , v/v) atmosphere. Repeated peaks of nitrogenase and growth were observed. Immunogold localization showed that in N2-fixing cultures, all cells, including those undergoing division, contained nitrogenase, and that the nitrogenase antigen was uniformly distributed throughout the cells without any preferential association with cellular structures. Rubisco was mainly located in carboxysomes of both N2-fixing and NOT-grown cells. Both N2-fixing and NOTgrown cells showed similar levels of PE, which was associated with the thylakoid membranes. GS antigen was distributed throughout the cells and the relative amounts of this enzyme, as well as its activity, were 20% higher in N2-fixing than in NOT-grown cultures. NO, uptake and NR systems were found to be NOT-inducible, with very low activities in N2-fixing cultures. The latter may be important in avoiding competition for Mo between nitrogenase and NR.

Localization of nitrogenase, glutamine synthetase (GS), phycoerythrin (PE) and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) was studied with immunocytochemical techniques in the cyanobiont and the free-living cultured isolate Nostoc 7801 of Anthoceros punctatus. In both cases nitrogenase was located in heterocysts only and was uniformly distributed within the cell. GS was located both in heterocysts and vegetative cells, with a uniform cellular distribution in each cell type. Whereas heterocysts of Nostoc 7801 had about twofold higher label than vegetative cells, labelling in heterocysts and vegetative cells of the cyanobiont was similar. While the GS content of the vegetative cells of the cyanobiont and Nostoc 7801 was comparable, the apparent GS content of the cyanobiont heterocysts was 60% less than that in Nostoc 7801 heterocysts. PE and RuBisCO were located in vegetative cells only. PE was located on thylakoid membranes and RuBisCO in the cytoplasm and carboxysomes. In each case the pattern of labelling in the cyanobiont and Nostoc 7801 was similar.

Spores of Frankia strain ACN lAG, immobilized in calcium alginate beads, germinated to produce colonies that increased in protein content and showed nitrogenase activity. Air dried immobilized spores remained viable for at least 15 days in dry condition, making the storage and transport of Frankia strains easy. This also opens the possibility of using beaded spores as inocula.