ALTERNATIVE RNA PRODUCTS FROM A RICE HOMEOBOX GENE

Three cDNA clones were isolated from rice, OSH42, OSH44 and OSH45, whi ch encode homeodomain sequences in the C-terminal region. The sequence s of these cDNAs differ in the N- and C-termini, but they share an ide ntical homeodomain and an acidic amino acid-rich region. The transcrip ts corresponding to these cDNAs are encoded by a single gene on rice c hromosome 8. Differential transcription initiation results in a large transcript comprised of exons 1 and 3-7 and a smaller transcript compr ised of exons 2-7. The larger transcript is constitutively expressed i n all tissues tested, while the smaller transcript is expressed in lea ves, stems and rachis but not in roots, flowers, or suspension callus cells. Alternative splicing also occurs at three different acceptor si tes 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 a cidic amino acid-rich region activates the expression of a reporter ge ne controlled by the GAL4 target sequence, indicating that it function s 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 co mpletely suppresses the transactivation activity of the acidic region. This suggests that the product of the larger transcript fails to acti vate the expression of the target gene(s) while the product of the sma ller transcript activates the expression of its target gene(s).