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

Franca Locatelli,Marcella Bracale,Flavio Magaraggia,Franco Faoro,Lucia A Manzocchi,Immacolata Coraggio

doi:10.1074/jbc.m000647200

Franca Locatelli, Marcella Bracale + Show 4 more

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https://doi.org/10.1074/jbc.m000647200

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Abstract

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.

Highlights

Mybs are a family of transcription factors widely represented in viruses, insects, mammals, and plants
The putative polypeptide encoded by the unspliced myb7 mRNA consists of an incomplete Myb domain followed by the leucine zipper: we named it Mybleu
Mybleu Has a Synergistic Effect on O2-driven Transactivation—The presence of a leucine zipper domain and the absence of both DNA binding and transactivation domains suggest that Mybleu might act as a dominant repressor, according to the results obtained with other incomplete transcription factors (44 – 47)

Summary

Introduction

Mybs are a family of transcription factors widely represented in viruses, insects, mammals, and plants. Some features of myb sequence indicate that it may be post-transcriptionally regulated; in particular, two unspliced introns are present in positions that are conserved with respect to other plant myb genes. The spliced form is instead predominant in rice roots during anoxia, a stress situation that generally inhibits splicing [14] These data suggest that post-transcriptional regulation controls the ratio between myb unspliced and spliced forms [7]. This hypothesis is supported by the observation that the 59 region of the first intron can direct the synthesis of an in-frame leucine zipper, a functional domain present in several transcription factors [15,16,17,18]. Recent data suggest that a bZIP protein functionally similar to O2 may exist in rice [22]

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