ALTERNATIVE RNA PRODUCTS FROM A RICE HOMEOBOX GENE

Three cDNA clones were isolated from rice, OSH42, OSH44 and OSH45, which encode homeodomain sequences in the C-terminal region. The sequences of these cDNAs differ in the N- and C-termini, but they share an identical homeodomain and an acidic amino acid-rich region. The transcripts corresponding to these cDNAs are encoded by a single gene on rice chromosome 8. Differential transcription initiation results in a large transcript comprised of exons 1 and 3-7 and a smaller transcript comprised of exons 2-7. The larger transcript is constitutively expressed in all tissues tested, while the smaller transcript is expressed in leaves, stems and rachis but not in roots, flowers, or suspension callus cells. Alternative splicing also occurs at three different acceptor sites in intron 6 in all tissues tested. The GAL4 DNA-binding domain of yeast was used to study the function of various protein domains. The acidic amino acid-rich region activates the expression of a reporter gene controlled by the GAL4 target sequence, indicating that it functions as a transactivation domain. The larger transcript encodes a unique alanine and glycine-rich region on the hi-terminal side of the acidic region, which is not encoded by the smaller transcript. This region completely suppresses the transactivation activity of the acidic region. This suggests that the product of the larger transcript fails to activate the expression of the target gene(s) while the product of the smaller transcript activates the expression of its target gene(s).