HD-Zip proteins of families I and II from rice: interactions and functional properties

Proteins of the closely related homeodomain-leucine zipper (HD-Zip) familie s I and II in plants are putative transcription factors that interact with similar pseudopalindromic DNA recognition sites. We have previously describ ed the Oshox1 gene from rice, which encodes an HD-Zip II protein. To identi fy further rice I-ID-Zip proteins, one-hybrid screens were per formed in ye ast strains containing a HIS3 reporter gene with upstream HD-Zip recognitio n sites. This resulted in the isolation of sis new cDNAs encoding HD-Zip pr oteins belonging to family I (Oshox4, -5, -6) or family II (Oshox2, -3, -7) . In transient assays, using rice suspension-cultured cells transformed by particle bombardment, we showed previously that Oshox1 can transcriptionall y repress the activity of reporter gene constructs with upstream HD-Zip bin ding sites. Here, we confirm the repression properties of Oshox1 by showing that the repression function can be conferred on a heterologous DNA-bindin g domain. This portable functional domain (residues 1-155) is located proxi mal to the HD-Zip domain. In yeast, the same region of the Oshox1 protein w as found to confer transcriptional activation instead of repression, pointi ng to the possibility that cell type-specific factors may determine the fun ctional properties of the Oshox1 protein in rice. Like Oshox1, another HD-Z ip family II protein (Oshox3) was also found to function as a transcription al repressor in rice cells. In contrast, two I-ID-Zip I family proteins (Os hox4 and -5) appeared to act as activators in both rice and yeast cells. Re sults of two-hybrid assays and electrophoretic mobility shift assays strong ly suggest that all HD-Zip proteins of families I and II can form homodimer s and also heterodimers with all I-ID-Zip proteins of the same family. Hete rodimerization across the HD-Zip families I and II apparently does not to o ccur.