Molecular cloning and characterization of OsPSK, a gene encoding a precursor for phytosulfokine-alpha, required for rice cell proliferation

We previously characterized an OsPSK cDNA encoding a precursor of phytosulf okine-alpha (PSK-alpha), a peptide plant growth factor. Southern blot analy sis suggested that OsPSK is a single-copy gene in rice, which we have isola ted and characterized. The OsPSK gene consists of one large intron and two exons. The 5-amino acid PSK-alpha sequence located close to the COOH-termin us of the precursor is encoded in the second exon. A putative TATA box was found at position -68 with respect to the transcription initiation site. Up stream of this sequence, several potential regulatory elements, including o ne CAAT-box, three CCAAT-boxes, one enhancer core-like sequence, and three E-boxes could be identified. By constructing plasmids with various lengths of the 5'-upstream regions of the OsPSK gene fused to the coding sequence f or bacterial beta -glucuronidase (GUS), we demonstrated a region 1.9 kb ups tream of the transcription initiation point, which contains most of the put ative 5'-regulatory elements, to be sufficient for maximal-level GUS expres sion in transformed rice Oc cells. The promoter of the OsPSK gene gave sign ificantly higher levels of GUS expression than the CaMV 35S promoter. These results suggest that the OsPSK promoter could be useful for the constituti ve expression of a foreign gene at high levels in transformed rice culture cells. Northern blot analyses suggest that the expression of OsPSK is reinf orced by auxin and cytokinin.