INDEPENDENCE OF CARBON AND NITROGEN CONTROL IN THE POSTTRANSLATIONAL REGULATION OF NITRATE TRANSPORT IN THE CYANOBACTERIUM SYNECHOCOCCUS SP. STRAIN PCC-7942

Nitrate transport by Synechococcus sp, strain PCC 7942 cells was inhib ited by ammonium and by inhibitors of CO2 fixation, Ammonium assimilat ion inhibitors, such as L-methionine D,L-sulfoximine, were known to pr event the negative effects of ammonium and of inhibitors of CO2 fixati on on nitrate uptake, leading to propose that CO2 fixation was require d to counteract the feed-back inhibition of nitrate assimilation. In N R-less mutants, L-methionine D,L-sulfoximine prevented the negative ef fects of ammonium on nitrate transport, but not always prevented those of inhibiting CO2 fixation, The carboxyterminal domain of the NrtC su bunit of the nitrate transporter has recently been identified as a reg ulatory domain involved in N-control. The mutant strain NC2, construct ed by deleting the 3' portion of nrtC, showed high nitrate transport a ctivity insensitive to ammonium but sensitive to inhibitors of CO2 fix ation, These findings indicate that the C-control and the N-control of nitrate transport are independent at both the physiological and the m olecular level. (C) 1998 Federation of European Biochemical Societies.