Regulation of nitrate reductase levels in the cyanobacteria Anacystis nidulans, Anabaena sp. strain 7119, and Nostoc sp. strain 6719

Abstract

The effect of the N source on cellular activity of ferredoxin-nitrate reductase in different cyanobacteria was examined. In the unicellular species A. nidulans, nitrate reductase was repressed in the presence of NH4, but de novo enzyme synthesis occurred in media containing either NO3 or no N source, indicating that NO3 was not required as an obligate inducer. Nitrate reductase in A. nidulans was freed from NH4 repression by L-methionine-D,L-sulfoximine, an irreversible inhibitor of glutamine synthetase. NH4 promoted repression appears to be indirect; NH4 must be metabolized through glutamine synthetase to be effective in the repression of nitrate reductase. Unlike the situation in A. nidulans, NO3 appeared to play an active role in nitrate reductase synthesis in the filamentous N2-fixing strains Anabaena sp. strain 7119 and Nostoc sp. strain 6719, with NH4 acting as an antagonist with regard to NO3.