The tomato I‐box binding factor LeMYBI is a member of a novel class of Myb‐like proteins

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<jats:title>Summary</jats:title><jats:p>The <jats:italic>RBCS3A</jats:italic> gene of tomato belongs to a small gene family consisting of five members. Although the <jats:italic>RBCS1</jats:italic>, <jats:italic>RBCS2</jats:italic> and <jats:italic>RBCS3A</jats:italic> promoters contain closely related <jats:italic>cis</jats:italic> regulatory sequences, the expression patterns of the genes are different. Whereas the <jats:italic>RBCS1</jats:italic> and <jats:italic>RBCS2</jats:italic> genes are expressed in both leaves and young fruit, the <jats:italic>RBCS3A</jats:italic> promoter is highly active in leaves, but not in young fruit. This lack of transcription could be due to a mutation in the <jats:italic>RBCS3A</jats:italic> promoter creating the so‐called F‐box, a protein binding site located between the activating <jats:italic>cis</jats:italic> elements, the I‐box and G‐box. In order to identify proteins that bind to the <jats:italic>RBCS3A</jats:italic> I‐box/F‐box region, the yeast one‐hybrid system was used. One clone, LeMYBI was isolated which contains strong similarity to plant myb transcription factors. The encoded LeMYBI protein is at least 188 amino acids in length and contains two myb‐like domains located at the amino terminus and close to the carboxy terminus, separated by a negatively charged domain. The protein contains a SHAQKYF amino acid signature motif in the second myb‐like repeat, which is highly conserved in a number of recently identified plant myb‐related genes, thus defining a new class of plant DNA‐binding proteins. LeMYBI binds specifically to the I‐box sequence of the <jats:italic>RBCS1</jats:italic>, <jats:italic>RBCS2</jats:italic> and <jats:italic>RBCS3A</jats:italic> promoters, therefore representing the first cloned I‐box binding factor. LeMYBI acts as a transcriptional activator in yeast and plants, and binds to the I‐box with a DNA‐binding domain located in the carboxyterminal domain.</jats:p>

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