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Nitrate inhibited nitrogenase synthesis and heterocyst devel- opment in the cyanobacterium Anabaena sp. strain PCC 7120. Inhibition of dinitrogen fixation by nitrate did not take place, however, in nitrate reductase-deficient derivatives of this strain. Hybridization of total RNA isolated from cells grown on different nitrogen sources with an internal fragment of the nifD gene showedthatregulationofnitrogenaseactivitybynitrate isexerted throughanegativecontrolofthenitrogenasemRNA levels.
Cyanobacteriaarephototrophiceubacteriathatcarryout oxygenic (plant-type) photosynthesis. Many filamentous cy- anobacteria,suchasthoseofthegenusAnabaena,areablet o fixdinitrogeninspecializedcelscalledheterocyststha tde- velopatsemi-regularintervalsalongthefilament.Heterocysts provideamicroaerobicenvironmentadequatefortheexpre s- sion and function of nitrogenase, the enzyme complex re- sponsiblefordinitrogenfixation.Boththedifferentiat ionofa vegetativecelintoaheterocystandtheexpressionofnitr o- genase activity are inhibited by ammonium (6, 14, 18) with very few exceptions (3, 18). As has been shown for the nitrogenase structural genes (nifIDK) and for some other genesrelatedtodinitrogenfixation,repressionbyammon ium operatesthroughregulationofthemRNA content(1,2,9,
15,19).Inhibitionofheterocystdevelopmentandofnitro gen- asesynthesisbynitratehasalsobeendescribed,butdiffe rent degreesofinhibitionhavebeenreportedfordifferentstr ains (3,9,14,16,17)andsomeauthorshavesuggestedthatnitra te doesnotsignificantlyaffectnitrogenasemRNA levels(9). Anotherquestionofinterestiswhetherornotnitrateredu c- tion is required for the inhibition by nitrate of dinitrogen fixation to be operative. Data aimed to clarify the nature of
This work was supported by grant no. B1089-0527 from the Comisi6nInterministerialdeCienciayTecnologia(Spain ).
2Present address: Division of Biology, Kansas State University, 348AckertHall,Manhattan,KS 66506.
theinhibitionbynitrateofdinitrogenfixationinafilam entous cyanobacteriumarepresentedinthiscommunication.
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"The coral itself has more energy for growth because the reduced zooxanthellae density requires less energy."
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