AMMONIUM-SENSITIVE PROTEIN-KINASE ACTIVITY IN PLASMA-MEMBRANES OF THECYANOBACTERIUM ANACYSTIS-NIDULANS

Cytoplasmic membranes prepared from nitrate-grown Anacystis nidulans c ells exhibit a Mg2+-dependent protein kinase activity able to phosphor ylate in vitro plasma membrane polypeptides with molecular masses of 9 8, 93, 83, 47, 44 and 31 kDa. The protein kinase activity was inhibite d in cytoplasmic membrane preparations from nitrate-grown cells which had been exposed to ammonium for 5 min. Parallely, ammonium exposure a lso resulted in a more than two-fold activation of an alkaline phospha tase activity present in the soluble fraction. These results are discu ssed in relation to the well-known inhibition by ammonium of nitrate t ransport activity, and a hypothesis for the regulatory mechanism invol ved is presented.