Characterization of the DNA-Binding Properties of the Cyanobacterial Transcription Factor NtcA
Abstract: Nitrogen is an essential building block of proteins and nucleic acids and, therefore, crucial for the biosphere. Nearly 79 % of the air consists of nitrogen, but in the form of nitrogen gas (N2), which cannot be utilized by most organisms. Nitrogen-fixing microorganisms such as cyanobacteria have a central role in supplying biologically useful nitrogen to the biosphere. Therefore, it is important to achieve further understanding of control mechanisms involved in nitrogen fixation and related processes. This thesis concerns different molecular aspects of the transcription factor NtcA from the heterocystous cyanobacterium Anabaena PCC 7120. Apart from performing oxygenic photosynthesis, Anabaena PCC 7120 is also capable of fixing nitrogen. NtcA is a protein regulating transcription of a wide range of genes and in particular genes involved in cyanobacterial global nitrogen control. NtcA binds as a dimer to the promoter regions of target genes such as those involved in nitrogen fixation and heterocyst differentiation. NtcA from Anabaena PCC 7120 was heterologously expressed in E. coli and a high yield of recombinant protein was achieved through purification by Ni-IMAC chromatography. The purified NtcA was used to examine DNA binding motifs preferred by NtcA in vitro using a semi-random library of DNA sequences. The preferred binding sequence for NtcA is TGTA – N8 – TACA and at least five of the bases in the palindromic binding site are necessary for binding. Differences in the consensus sequence in vivo may reflect variations in the structural conformation of NtcA under various physiological conditions. Since an earlier study suggested redox-regulated NtcA-DNA binding the role of the two cysteine residues of NtcA were investigated. Binding studies using three mutants, Cys157Ala, Cys164Ala, and Cys157Ala / Cys164Ala, demonstrated that all these NtcA variants bind to DNA with a slightly higher affinity in the presence of the reducing agent DTT. The studies indicate that the binding mechanism is not dependent on a conformational change of NtcA caused by breaking of intra-molecular disulfide bonds. Crystallization followed by structural studies rendered a partial crystal structure of NtcA. The structure verifies that NtcA is a dimeric protein. Each subunit has three domains: the N-terminal domain, a dimerization helix connecting the N-terminal domain with the C-terminal domain, as well as making up the dimer interface, and a C-terminal domain including the DNA binding helix-turn-helix motif.Furthermore, an NtcA binding site was found in the promoter region of the hupSL gene, encoding an uptake hydrogenase in Nostoc punctiforme (ATCC 29133), indicating that yet another gene is transcriptionally controlled by NtcA, thereby further emphasizing the multifaceted role of NtcA in cyanobacteria.
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