Year of Publication

2013

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Agriculture, Food and Environment

Department

Plant and Soil Sciences

First Advisor

Dr. Ling Yuan

Second Advisor

Dr. Robert L. Houtz

Abstract

Herewithin, two transcription factor (TF) regulatory complexes were investigated. A bHLH–MYB–WDR (BMW) DNA-binding complex from maize was the first complex to be studied. R, a maize bHLH involved in the activation of genes in the anthocyanin pathway, had been characterized to indirectly bind DNA despite the presence of a functional DNA-binding domain. Findings presented here reveal that this is only partially correct. Direct DNA-binding by R was found to be dependent upon two distinct dimerization domains that function as a switch. This switch-like mechanism allows R to be repurposed for the activation of promoters of differing cis-element structure.

The second regulatory complex studied was of the Arabidopsis thaliana MIKC-MADS TF family. For many TFs, DNA-binding site recognition is relatively straightforward and very sequence specific, while others exhibit relaxed sequence specificity. MADS-domain TFs are one family of TFs with a wider range of cis-element sequences. Though consensus cis-element sequences have been determined for various MADS-domains, correctly predicting and identifying biologically functional cis-elements has been a challenge. In order to study the influence of nucleobase associations within the cis-element, a DNA-Protein Interaction (DPI)-ELISA method was modified and optimized to screen a panel of specific probes. Screening of the SEP3 homodimer against a panel of sequential, palindromic probes revealed that nucleobases in position -1:+1 of the CArG-box influence binding strength between the MADS-domain and DNA. Additionally, the specificity of AGL15 towards CT-W6-AG forms was discovered to be determined by the functional groups present in the minor groove at position -4:+4 using inosine:cytosine (I:C) base pairs.

Finally, the FLC–SVP MADS-domain heterodimer, bound to a native cis-element, was modeled and binding simulated using molecular dynamics. In conjunction with simulations of AGL15 and SEP3 homodimers, a potential binding mechanism was identified for this unique heterodimer. DNA sequence recognition by the MADS-domain was found to occur asymmetrically. In the case of the FLC–SVP heterodimer, the direction of asymmetrical DNA-binding in heterodimers was found to be fixed. Furthermore, the molecular dynamics simulations provided insight towards understanding the results generated from previous DPI-ELISA experiments, which should provide an improved means for predicting biologically significant CArG-boxes around genes.

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