Synthesis and proteolytic degradation of nitrogenase in cultures of the unicellular cyanobacterium Gloeothece strain ATCC 27152

In cultures of the unicellular cyanobacterium Gloeothece sp, ATCC 27152 gro wing under alternating 12 h light and 12 h darkness, nitrogenase activity a ppears as cultures enter the dark phase. Synthesis of both component protei ns of nitrogenase commences immediately prior to the appearance of activity and continues until about 8 h into the period of darkness. The two compone nts (Fe-protein and MoFe-protein) are synthesized in a molar ratio of about 3:1. Degradation of the nitrogenase proteins starts as early as 4 h into t he dark period and increases markedly as cultures enter the light phase. As a result, both nitrogenase proteins are completely absent from cultures du ring most of the light phase. In contrast, all of the other proteins invest igated appeared to be present throughout the cycle of alternating light and darkness. Degradation of nitrogenase depends upon protein synthesis during the last 6 h of darkness and is prevented by addition of protease inhibito rs. Two proteins, of M-r 47 000 and 29000, are specifically synthesized dur ing this period and it is possible that they have a role in nitrogenase deg radation. Proteolytic activity of extracts of Gloeothece, measured as the a bility to degrade azocasein, increased markedly during the early part of th e light period, but this increase did not depend on protein synthesis; This activity does not therefore correspond to that specifically involved in ni trogenase catabolism. though it may act on initial breakdown products gener ated by a nitrogenase-specific degradative system. A phycobiliprotein appea rs to act as a temporary store of the degradation products of nitrogenase.