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

John P. H. Reade^1,2,

¹ Biochemistry Research Group, School of Biological Sciences, University of Wales Swansea, Swansea SA2 8PP, UK
² Institute of Biological Sciences, University College of Wales, Sir George Stapledon Building, Aberystwyth SY23 3DD, UK

Author for correspondence: John R. Gallon. Tel: -44 1792 295376. Fax: -44 1792 295447. e-mail: j.r.gallon@swansea.ac.uk

ABSTRACT

In cultures of the unicellular cyanobacterium Gloeothece sp. ATCC 27152 growing under alternating 12 h light and 12 h darkness, nitrogenase activity appears as cultures enter the dark phase. Synthesis of both component proteins of nitrogenase commences immediately prior to the appearance of activity and continues until about 8 h into the period of darkness. The two components (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 the dark period and increases markedly as cultures enter the light phase. As a result, both nitrogenase proteins are completely absent from cultures during most of the light phase. In contrast, all of the other proteins investigated 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 inhibitors. Two proteins, of M_r 47000 and 29000, are specifically synthesized during this period and it is possible that they have a role in nitrogenase degradation. Proteolytic activity of extracts of Gloeothece, measured as the ability to degrade azocasein, increased markedly during the early part of the light period, but this increase did not depend on protein synthesis. This activity does not therefore correspond to that specifically involved in nitrogenase catabolism, though it may act on initial breakdown products generated by a nitrogenase-specific degradative system. A phycobiliprotein appears to act as a temporary store of the degradation products of nitrogenase.

Present address: Crop and Environmental Research Centre, Harper Adams University College, Newport, Shropshire TF10 8NB, UK.

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