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Biochim Biophys Acta. 2006 Jun;1759(6):308-18. Epub 2006 May 13.

Directed evolution of plant basic helix-loop-helix transcription factors for the improvement of transactivational properties.

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Department of Plant and Soil Sciences, University of Kentucky, Cooper and University Drives, Lexington, KY 40546, USA.


Myc-RP from Perilla frutescens and Delila from Antirrhinum majus, two plant basic helix-loop-helix transcription factors (bHLH TFs) involved in the flavonoid biosynthetic pathway, have been used for the improvement of transactivational properties by directed evolution. Through two rounds of DNA shuffling, Myc-RP variants with up to 70-fold increase in transcriptional activities have been identified using a yeast transactivation system. In a tobacco protoplast transient expression assay, one of the most improved variants, M2-1, also shows significant increase of transactivation. The majority of resulting mutations (approximately 53%) are localized in the acidic (activation) domains of the improved Myc-RP variants. In variant M2-1, three of the four mutations (L301P/N354D/S401F) are in the acidic domain. The fourth mutation (K157M) is localized to a helix within the N-terminal interaction domain. Combinatorial site-directed mutagenesis reveals that, while the acidic domain mutations contribute modestly to the increase in activity, the K157M substitution is responsible for 80% of the improvement observed in variant M2-1. The transactivation activity of the K157M/N354D double mutant is equal to that of M2-1. These results suggest that the interaction domain plays a critical role in transactivation of these bHLH TFs. Delila variants have also been screened for increased activities toward the Arabidopsis chalcone synthase (CHS) promoter, a pathway promoter that responds weakly to the bHLH TFs. Variants with increased activity on the CHS promoter, while maintaining wildtype-level activities on the naturally responsive dihydroflavonol reductase promoter, have been obtained. This study demonstrates that functional properties of TFs can be modified by directed evolution.

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