Figure 2. PAL as a Case Study ofMultipleMechanisms Regulating Gene Expression and Protein Abundance. (A) PAL is directly repressed through binding of various TALE family transcription factors (TFs; e.g., BREVIPEDICELLUS) to the DNA knotted 1 (KN-1) motif, as well as by binding of subgroup 4 R2R3 MYBs (e.g., MYB4) to the AATAGTT motif. (B) PAL is indirectly regulated through BLH6–KNAT7-mediated repression of the HD-Zip III TF REVOLUTA. HB15 represses the expression of the master switches SND1 and NST2, which are activators of PAL expression. (C) Kelch F-box proteins ubiquitinate PAL to target it to the 26S proteasome for proteolysis. Other PAL regulatory pathways, including metabolic feedback repression (caffeic acid, cinnamic acid), environmental factors, and physical interactions with other phenylpropanoid enzymes (C4H) are beyond the scope of this figure but are reviewed in [44]. Abbreviations: BLH, BEL1-like homeodomain; HB, homeobox; KFB, kelch F-box; KNAT, knotted1-like TALE homeodomain; NST, NAC secondary wall thickening promoting factor; PAL, phenylalanine ammonia lyase; SND, secondary wall-associated NAC domain protein; Ub ubiquitin. This figure was created using BioRender (https://biorender.com/).
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