Ammonium-Sensitive Protein-Kinase Activity in Plasma-Membranes of the Cyanobacterium Anacystis-Nidulans

Cytoplasmic membranes prepared from nitrate-grown Anacystis nidulans cells exhibit a Mg2+-dependent protein kinase activity able to phosphorylate in vitro plasma membrane polypeptides with molecular masses of 98, 93, 83, 47, 44 and 31 kDa. The protein kinase activity was inhibited in cytoplasmic membrane preparations from nitrate-grown cells which had been exposed to ammonium for 5 min. Parallely, ammonium exposure also resulted in a more than two-fold activation of an alkaline phosphatase activity present in the soluble fraction. These results are discussed in relation to the well-known inhibition by ammonium of nitrate transport activity, and a hypothesis for the regulatory mechanism involved is presented.