REGULATION BY CYANATE OF THE GENES INVOLVED IN CARBON AND NITROGEN ASSIMILATION IN THE CYANOBACTERIUM SYNECHOCOCCUS SP STRAIN PCC 7942

A mutant (M45) of the cyanobacterium Synechococcus sp, strain PCC 7942 , which is defective in active transport of nitrate, was used for the studies of the nitrogen regulation of the genes involved in nitrate an d CO2 assimilation, In a medium containing 30 mM nitrate as the nitrog en source, M45 grew under constant stress of nitrogen deficiency and a ccumulated a five-times-larger amount of the transcript of nirA, the g ene for nitrite reductase, compared with nitrate-grown wild-type cells , By contrast, the level of the transcript of rbcL, the gene for the l arge subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase, was 4 0% of the wild type level, Addition of ammonium to the culture of M45 abolished the accumulation of the nirA transcript and stimulated the a ccumulation of the rbcL transcript, showing that ammonium repressed an d activated the transcription of nil A and rbcL, respectively, Glutami ne, the initial product of ammonium fixation, also showed negative and positive effects on nirA and rbcL, respectively, One of the metabolit es of glutamine, carbamoylphosphate, and its decomposition product, cy anate, were found to repress nirA and also to markedly activate rbcL, Cyanate negatively regulated another ammonium-repressible gene, glnA, but had no effect on the psbAl and rpsI genes, The effects of cyanate were not ascribable to the ammonium and CO2 resulting from its decompo sition, These findings suggested that cyanate may act as a regulator o f the ammonium-responsive genes involved in carbon and nitrogen assimi lation in the cyanobacterium.