Regulation of urease activity in the cyanobacterium Anabaena doliolum

Abstract

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 N2, NO3−, or urea. Ammonium grown cell showed repression in urease activity which was freed by l-methionine, dl-sulphoximine (MSX), an inhibitor of glutamine synthetase. NH4+, must therefore, be metabolised through glutamine synthetase before repressing the urease activity. NH4+ (up to 10 mM) did not inhibit the urease activity in vitro, suggesting that NH4+ does not inactivate the enzyme but represses its biosynthesis. When NH4+ 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. Ni2+ 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 Ni2+ showed a higher level of urease activity as compared to chloramphenicol treated cells only, suggesting the antagonistic nature of Ni2+ against chloramphenicol mainly at the level of synthesis/activity of proteases.