Transcriptional repression by AtMYB4 controls production of UV-protecting sunscreens in Arabidopsis

An Arabidopsis thaliana line that is mutant for the R2R3 MYB gene, AtMYB4, shows enhanced levels of sinapate esters in its leaves. The mutant line is more tolerant of UV-B irradiation than wild type. The increase in sinapate ester accumulation in the mutant is associated with an enhanced expression of the gene encoding cinnamate 4-hydroxylase, which appears to be the princ ipal target of AtMYB4 and an effective rate limiting step in the synthesis of sinapate ester sunscreens. AtMYB4 expression is downregulated by exposur e to UV-B light, indicating that derepression is an important mechanism for acclimation to UV-B in A,thaliana, The response of target genes to AtMYB4 repression is dose dependent, a feature that operates under physiological c onditions to reinforce the silencing effect of AtMYB4 at high activity. AtM YB4 works as a repressor of target gene expression and includes a repressio n domain. It belongs to a novel group of plant R2R3 MYB proteins involved i n transcriptional silencing, The balance between MYB activators and repress ors on common target promoters may provide extra flexibility in transcripti onal control.