Adenylylation/Deadenylylation Control of the Glutamine Synthetase of Rhodopseudomonas capsulata

Previous attempts to demonstrate adenylylation/deadenylylation control of glutamine synthetase from photosynthetic organisms have given negative results; we have reexamined the enzyme of Rhodopseudomonas capsulata in this connection. Studies on other systems have shown that adenylylated and deadenylylated forms of glutamine synthetase respond differently to Mg 2+ in the γ‐glutamyltransferase assay, and this was observed with the R. capsulata enzyme when its adenylylation state was stabilized by addition of cetyltrimethylammonium bromide to cells before they were harvested for preparation of extracts. The relative activities of glutamine synthetase in the presence and absence of added Mg 2+ is an index of adenylylation state and on the basis of this and other criteria, the enzyme in R. capsulata cells grown with excess NH + 4 as N source is largely adenylylated, whereas the deadenylylated form predominates when the organism is grown on N 2 . The results of experiments on the effects of snake venom phosphodiesterase on response of partially purified glutamine synthetase (from R. capsulata cells grown on NH + 4 ) to metal ion supplementation (Mg 2+ and Mn 2+ ) provided confirmatory evidence for adenylylation. Conditions are described for demonstration of regulation of R. capsulata glutamine synthetase adenylylation state, in vivo , by light intensity. When the N source for growth is glutamate, the extent of adenylylation decreases with increase of light intensity. In cells subjected to a sudden shift‐down in light intensity, the extent of glutamine synthetase adenylylation increases rapidly; conversely, a shift‐up in light intensity causes, after a lag, a decrease in the degree of adenylylation. The light intensity effects are interpreted to reflect regulation of adenylylation/deadenylylation by the ‘energy state’ of the cell.