Regulation of urease activity in the cyanobacterium Anabaena doliolum

The effect of nitrogen sources on the activity of urease has been studied in the cyanobacterium Anabaena doliolum . De novo synthesis of urease occurred in the absence of an added nitrogen source, and the enzyme activity was almost equal when the medium contained either N 2 , NO 3 − , or urea. Ammonium grown cell showed repression in urease activity which was freed by l ‐methionine, dl ‐sulphoximine (MSX), an inhibitor of glutamine synthetase. NH 4 + , must therefore, be metabolised through glutamine synthetase before repressing the urease activity. NH 4 + (up to 10 mM) did not inhibit the urease activity in vitro, suggesting that NH 4 + does not inactivate the enzyme but represses its biosynthesis. When NH 4 + grown cells were transferred to medium containing urea, the ability to hydrolyse urea required approximately 5 to 6 h for maximal expression. Chloramphenicol (50 μg ml −1 ) completely prevented the rise in urease activity. Ni 2+ at concentrations of 0.01 and 0.05 μM enhanced the urease activity; however, the addition of 10mM citrate to the medium resulted in a drop in the increase in urease activity. The cells treated with chloramphenicol in combination with Ni 2+ showed a higher level of urease activity as compared to chloramphenicol treated cells only, suggesting the antagonistic nature of Ni 2+ against chloramphenicol mainly at the level of synthesis/activity of proteases.