2 GENES THAT ENCODE RIBOSOMAL-PROTEIN-S6 KINASE HOMOLOGS ARE INDUCED BY COLD OR SALINITY STRESS IN ARABIDOPSIS-THALIANA

We have isolated two closely related cDNA clones (cATPK19 and cATPK6) with homology to protein-serine/threonine kinases from Arabidopsis tha liana using the polymerase chain reaction (PCR). The deduced amino aci d sequences of the ATPK19 and ATPK6 contain all 11 conserved regions o f the catalytic domain of protein kinases, and have homology to p70 ri bosomal S6 kinases (52%). ATPK19 and ATPK6 have putative PEST regions in their N- and C-terminal regions, and these regions also contain put ative phosphorylation sites that are recognized by casein kinases or p roline-directed protein kinases such as cdc2, MAP kinase, and p54 MAP- 2 kinase (SAPK). The transcription levels of the ATPK19 and ATPK6 gene s rapidly and markedly increased when plants were subjected to cold or high-salt stresses. These observations suggest that ATPK19 and ATPK6 may function in the adaptation of plant cells to cold or high-salt con ditions, providing an understanding of the role of protein phosphoryla tion in plant responses to environmental stresses.