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Journal of Bacteriology, November 2004, p. 7337-7343, Vol. 186, No. 21
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.21.7337-7343.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

The NtcA-Dependent P₁ Promoter Is Utilized for glnA Expression in N₂-Fixing Heterocysts of Anabaena sp. Strain PCC 7120

Ana Valladares, Alicia M. Muro-Pastor, Antonia Herrero, and Enrique Flores^*

Instituto de Bioquímica Vegetal y Fotosíntesis, Consejo Superior de Investigaciones Científicas-Universidad de Sevilla, Seville, Spain

Received 15 April 2004/ Accepted 19 July 2004

Expression of the glnA gene encoding glutamine synthetase, a key enzyme in nitrogen metabolism, is subject to a variety of regulatory mechanisms in different organisms. In the filamentous, N₂-fixing cyanobacterium Anabaena sp. strain PCC 7120, glnA is expressed from multiple promoters that generate several transcripts whose abundance is influenced by NtcA, the transcription factor exerting global nitrogen control in cyanobacteria. Whereas RNA_I originates from a canonical NtcA-dependent promoter (P₁) and RNA_II originates from a ⁷⁰-type promoter (P₂), RNA_IV is influenced by NtcA but the corresponding promoter (P₃) does not have the structure of NtcA-activated promoters. Using RNA isolated from Anabaena filaments grown under different nitrogen regimens, we observed, in addition to these transcripts, RNA_V, which has previously been detected only in in vitro transcription assays and should originate from P₄. However, in heterocysts, which are differentiated cells specialized in N₂ fixation, RNA_I was the almost exclusive glnA transcript. Analysis of P_glnA::lacZ fusions containing different fragments of the glnA upstream region confirmed that fragments carrying P₁, P₂, or P₃ and P₄ have the ability to promote transcription. Mutation of the NtcA-binding site in P₁ eliminated P₁-directed transcription and allowed increased use of P₂. The NtcA-binding site in the P₁ promoter and binding of NtcA to this site appear to be key factors in determining glnA gene expression in vegetative cells and heterocysts.

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* Corresponding author. Mailing address: Instituto de Bioquímica Vegetal y Fotosíntesis, Centro de Investigaciones Científicas Isla de la Cartuja, Avda. Américo Vespucio s/n, E-41092 Seville, Spain. Phone: 34 95 448 9523. Fax: 34 95 446 0065. E-mail: eflores{at}ibvf.csic.es.

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Journal of Bacteriology, November 2004, p. 7337-7343, Vol. 186, No. 21
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.21.7337-7343.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

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