ATPK1, A NOVEL RIBOSOMAL-PROTEIN KINASE GENE FROM ARABIDOPSIS .1. ISOLATION, CHARACTERIZATION, AND EXPRESSION

Two protein kinase genes (atpk1 and atpk2) were isolated from Arabidop sis thaliana genomic DNA with a probe generated by polymerase chain re action (PCR) using oligonucleotide primers encoding conserved eukaryot ic protein kinase sequences. atpk1 and atpk2 are organized in a head-t o-tail tandem array on chromosome 3 and have about 80% nucleotide sequ ence identity. atpk1 encodes a hydrophilic polypeptide of 465 amino ac ids, M(r)=52,554. The centrally located catalytic domain contains all the conserved residues characteristic of eukaryotic protein kinases, w ith greatest similarity to the catalytic domains of 70-kDa ribosomal S 6 protein kinase, protein kinase C, and protein kinase A. The C-termin al 75 residues also show homology to protein kinase C and S6 protein k inase. In contrast, the N-terminal 130 residues have no homology to an y known protein, and thus may represent a new class of protein kinase regulatory domain. Other motifs found in the Atpk1 protein include two putative autophosphorylation sites, a pseudosubstrate site, two acidi c domains, a lysine-rich domain, and two putative PEST sequences, whic h may contribute to the regulation of protein kinase activity. RNA-blo t hybridization showed that atpk1 encoded a 1.8-kb mRNA Analysis of at pk1 promoter/beta-glucuronidase reporter gene fusions in transgenic pl ants showed that atpk1 was expressed in all tissues and at all develop mental stages, with the strongest expression observed in metabolically active tissues, suggesting that atpk1 is involved in the control of p lant growth and development. The first intron of atpk1 functions as an enhancer in atpk1 expression.