Cultures of the cyanobacterium Anacystis nidulans were grown under conditions of Mg2+-limitation and Mg2+-excess. Cell-free extracts, obtained after sonication and centrifugation (45000 g, 60 min), were analysed on polyacrylamide gels. Mg2+-limited cells, in which cell division was inhibited, accumulated a protein of molecular weight 50 103. Only small amounts of this protein were detected in non-Mg2+-lirnited cultures. A protein of molecular weight 36 103, found in non-Mg2+-limited cells, was not detected in Mg2+-limited cells. When a Mg2+ shift-up from 5 m to 1 mM was carried out in a chemostat, synthesis of the 36 103 protein was initiated and the amount of the 50 103 protein decreased whilst the main protein pattern remained unaltered. The possibility that the two proteins are involved in cell division is discussed.
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