The Product of the Rice myb7 Unspliced mRNA Dimerizes with the Maize Leucine Zipper Opaque2 and Stimulates Its Activity in a Transient Expression Assay

myb7 mRNA is present in rice in spliced and unspliced
forms, splicing being enhanced by anoxia. The protein
(Mybleu) encoded by the unspliced mRNA is composed
of an incomplete Myb domain followed by a leucine zipper;
however, it lacks canonical sequences for DNA
binding, transcriptional activation, and nuclear localization.
We show here that in transiently transformed
tobacco protoplasts, Mybleu is able to enhance the transcriptional
activity of the maize leucine zipper Opaque2
on its target b32 promoter. The Mybleu transactivation
effect is strictly dependent on the presence of Opaque2
and is driven by Mybleu-Opaque2 heterodimers. Mybleu
is located in the nucleus, both in rice and in transformed
tobacco protoplasts. In rice, the protein is expressed in
regions corresponding to undifferentiated cells of roots
and coleoptiles. Therefore, myb7 mRNA encodes, depending
on its splicing, two transcription factors belonging
to separate classes. One of them, Mybleu, has
novel structural characteristics, suggesting the existence
of new mechanisms acting in the activation of
transcription.