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.

Maturation and translation mechanisms involved in the expression of a myb gene of rice.

We have isolated two overlapping cDNAs coding for a MYB-related protein expressed in aerobic and anaerobic rice (Oryza sativa) roots and coleoptiles. Analysis of their sequences reveals some peculiar features, suggesting the presence of post-transcriptional regulation events: an upstream ORF, two unspliced introns and a putative leucine zipper in the ORF coded by the unspliced RNA. Transient expression in protoplasts indicates that the upstream ORF inhibits expression of a downstream coding sequence. Finally, we demonstrated that anoxia, in roots, increases the ratio between the spliced and the unspliced mRNA and affects the expression of other myb-related genes.