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 phytosulfokine-alpha (PSK-alpha), a peptide plant growth factor. Southern blot analysis suggested that OsPSK is a single-copy gene in rice, which we have isolated 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-terminus 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. Upstream of this sequence, several potential regulatory elements, including one 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 for bacterial beta-glucuronidase (GUS), we demonstrated a region 1.9 kb upstream of the transcription initiation point, which contains most of the putative 5'-regulatory elements, to be sufficient for maximal-level GUS expression in transformed rice Oc cells. The promoter of the OsPSK gene gave significantly higher levels of GUS expression than the CaMV 35S promoter. These results suggest that the OsPSK promoter could be useful for the constitutive expression of a foreign gene at high levels in transformed rice culture cells. Northern blot analyses suggest that the expression of OsPSK is reinforced by auxin and cytokinin.