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Anabaena variabilis ATCC 29413 has one Mo nitrogenase that is made under oxic growth conditions in specialized cells called heterocysts and a second Mo nitrogenase that is made only under anoxic conditions in vegetative cells. The two largenif gene clusters responsible for these two nitrogenases are under the control of the promoter of the first gene in the operon,nifB1 ornifB2 . Despite differences in the expression patterns ofnifB1 andnifB2 , related to oxygen and cell type, the regions upstream of their transcription start sites (tss) show striking homology, including three highly conserved sequences (CS). CS1, CS2, and the region just upstream from the tss were required for optimal expression from thenifB1 promoter, but CS3 and the 5′ untranslated region (UTR) were not. Hybrid fusions of thenifB1 andnifB2 upstream regions revealed that the region including CS1, CS2, and CS3 ofnifB2 could substitute for the similar region ofnifB1 ; however, the converse was not true. Expression from thenifB2 promoter region required the CS1, CS2, and CS3 regions ofnifB2 and also required thenifB2 5′ UTR. A hybrid promoter that was mostlynifB2 but that had the region from about position −40 to the tss ofnifB1 was expressed in heterocysts and in anoxic vegetative cells. Thus, addition of thenifB1 promoter region (from about position −40 to the tss ofnifB1 ) in thenifB hybrid promoter supported expression in heterocysts but did not prevent the mostlynifB2 promoter from also functioning in anoxic vegetative cells.IMPORTANCE In the filamentous cyanobacteriumAnabaena variabilis , two Mo nitrogenase gene clusters,nif1 andnif2 , function under different environmental conditions in different cell types. Little is known about the regulation of transcription from the promoter upstream of the first gene of the cluster, which drives transcription of each of these two large operons. The similarity in the sequences upstream of the primary promoters for the twonif gene clusters belies the differences in their expression patterns. Analysis of thesenif promoters in strains with mutations in the conserved sequences and in strains with hybrid promoters, comprising parts fromnif1 andnif2 , provides strong evidence that each promoter has key elements required for cell-type-specific expression of thenif1 andnif2 gene clusters.

journal of bacteriology

Role of the <i>nifB1</i> and <i>nifB2</i> Promoters in Cell-Type-Specific Expression of Two Mo Nitrogenases in the Cyanobacterium Anabaena variabilis ATCC 29413

Role of the nifB1 and nifB2 Promoters in Cell-Type-Specific Expression of Two Mo Nitrogenases in the Cyanobacterium Anabaena variabilis ATCC 29413