PARENT SESSION
Posters P8C C4 and CAM. Abstracts (685-698)

Identification and expression analyses of rice three phosphoenolpyruvate carboxylase kinase genes. Hiroshi Fukayama^*,1, Stuart Sullivan², Mitsue Miyao¹, Hugh Nimmo², ¹ National Institute of Agrobiological Sciences, Tsukuba, Japan² Institute of Biomedical & Life Sciences, Glasgow, UK

ABSTRACT- Phosphorylation of phosphoenolpyruvate carboxylase (PEPC) plays an important role in controlling central cellular metabolism of higher plants. Three PEPC kinase (PPCK) genes, OsPPCK1, OsPPCK2 and OsPPCK3, were identified in the rice genome sequence database. In vitro transcription/translation of their full-length cDNAs followed by the assay of PPCK activity indicated that all three genes encode a functional PPCK protein. The OsPPCK2 gene has an intriguing feature, in that it has two transcription initiation sites. Transcription from the second initiation site gives rise to a translation product similar to those of OsPPCK1 and OsPPCK3, while transcription from the first site gives rise to a longer protein with an extension of 13 amino acid residues at the N terminus. The expression of three PPCK genes was studied by RT-PCR analyses, in which the two different transcripts of OsPPCK2 were analyzed together. Results indicated that OsPPCK1 and OsPPCK3 were mainly expressed in the roots and, albeit to a much lesser extent, in other organs, while OsPPCK2 was expressed in all organs examined. Only OsPPCK3 showed diurnal oscillation of expression in the leaves; it was specifically expressed in the night. On either nitrate addition or phosphorus starvation in the leaves, expression of OsPPCK2 was greatly induced, while expression of the other two genes was not affected. These results suggest that the three PPCK genes have different functions in rice plants. When the long and short transcripts of OsPPCK2 were analyzed separately, it was found that the two transcripts showed different expression characteristics. Organ specificity, response to phosphorus starvation, and diurnal pattern were different, although the long transcript was much less abundant than the short one in all cases. These results imply that OsPPCK2 might have dual functions, mediated through selection of the transcription initiation site, although the presence of two different translation products remains to be proved.

KEY WORDS: protein phosphorylation, rice, phosphoenolpyruvate carboxylase kinase, gene expression