PCMYB1, A NOVEL PLANT PROTEIN CONTAINING A DNA-BINDING DOMAIN WITH ONE MYB REPEAT, INTERACTS IN-VIVO WITH A LIGHT-REGULATORY PROMOTER UNIT

Light-regulatory unit 1 (LRU1) is necessary for and sufficient to medi ate light-dependent activation of the chalcone synthase (CHS) minimal promoter in Petroselinum crispum. This composite promoter unit consist s of at least two distinct cis-acting elements, designated ACE(CHS) an d MRECHS, both of which are required for right induction. The ACGT-con taining element ACE(CHS) interacts with common plant regulatory factor s (CPRFs) which belong to the basic region/leucine zipper (bZIP) class of transcription factors. Here, we demonstrate that MRECHS, originall y identified as an in vivo DNA footprint, is a MYB recognition element . This element possesses a functional core that is essential for light responsiveness and is specifically recognized by two distantly relate d MYB-like proteins: MYB305 and the never factor MYB1 from P. crispum. PcMYB1 was identified by both its specific binding to MRECHS in vitro and recognition of MRECHS in vivo. The deduced amino acid sequence re vealed that PcMYB1 contains only one MYB-like repeat. This portion of the protein constitutes the DNA-binding domain. Mutational analysis of PcMYB1 in combination with sequence comparison suggests the presence of a helix-turn-helix structure containing a recognition helix that is sufficient for sequence-specific binding. The structure of this disti nct MYB-like DNA-binding domain appears to be conserved in proteins fr om all three eukaryotic phyla.