Kinetics and regulation of urea uptake in Anabaena doliolum and Anacystis nidulans.

The apparent Km and Vmax values for urea uptake in Anabaena doliolum Bharadwaja were 250μM and 568nmol of urea (mg protein)-1hr-1. In comparison, Anacystis nidulans IU 625 showed lower affinity, Km being 400μM and Vmax 500nmol of urea (mg protein)-1hr-1. Urea uptake was optimal at pH 7.5, 15W m-2 illumination and 30°C. Maximum uptake was evident during the exponential growth phase. Cells pregrown on N2, urea or nitrate transported urea equally well, but there was essentially no uptake of urea in cells preincubated with ammonia for more than 24hr. Nevertheless, ammonia did not inhibit urea uptake by urea grown cells. But the uptake of ammonia was reduced in the presence of urea. N- deficiency invariably stimulated uptake of urea. Experiments under different metabolic conditions revealed that uptake is an active process driven mainly by cyclic photophosphorylation.