Apparent redundancy in myb gene function provides gearing for the control of flavonoid biosynthesis in antirrhinum flowers.

Two Myb-related transcription factors, Myb305 and Myb340, are expressed specifically in flowers of Antirrhinum. The proteins are structurally very similar throughout their DNA binding domains, implying that they bind to common target motifs. This binding has been demonstrated experimentally. Myb305 has been shown to activate the gene encoding the first enzyme of phenylpropanoid metabolism, phenylalanine ammonia-lyase. We show that Myb340 can also activate transcription from the phenylalanine ammonia-lyase gene promoter and that both transcription factors can activate two other genes involved in flavonoid metabolism, thereby linking early and later steps in plant secondary metabolism. Myb340 is a stronger activator than Myb305, but relatively more Myb305 than Myb340 protein is able to bind to target promoters when both proteins are synthesized in yeast or Escherichia coli, probably as a result of inhibition of Myb340 binding by phosphorylation. This means that Myb305 can compete with Myb340 to reduce its effective transcriptional activation when both transcription factors are expressed in the same cell. This competitive interaction has been demonstrated in plant cells. Expression patterns determined by in situ hybridization showed that the two transcription factors are expressed within the same cells of the flower and imply that the detailed specializations in function of these two apparently redundant transcription factors may be used to provide gears that adjust the rate of induction of secondary metabolism to floral development.