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MYB proteins contain a conserved DNA binding domain (MYB domain) and often function as transcription factors, playing roles in cell cycle regulation and/or development. Barley GAMYB functions in GA signaling in aleurone cells after seed germination. An analysis by Millar and Gubler (pages 705-721) of Arabidopsis with T-DNA insertional mutations in MYB33 and MYB65, which have high sequence similarity with barley GAMYB, revealed that these two genes have functionally redundant roles in pollen development. Neither single mutant displayed an obvious phenotype, but the double mutant showed premeiotic abortion of pollen development. The two genes contain a putative microRNA target sequence, and mutating this sequence in MYB33 resulted in an expanded pattern of gene expression, implying that the microRNA target sequence normally restricts gene expression. As shown on the cover, Arabidopsis transformed with MYB33 containing the mutated microRNA target exhibited pleiotrophic developmental defects, suggesting that the restriction of MYB33 expression is essential for proper plant development.
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