Cloning and analysis of a gene that encodes a homologue of protein kinase (PK) from Arabidopsis thaliana is reported. Oligodeoxyribonucleotides (oligos) corresponding to conserved regions in catalytic domains of various PKs were used for polymerase chain reaction (PCR) amplification with genomic DNA from A. thaliana as template, in an attempt to identify genes encoding PK in plants. We obtained several amplified DNA fragments that encoded part of a PK. We screened a genomic DNA library of A. thaliana with these oligos or PCR fragments as probes. Three genomic clones were obtained and one of them, named Atpk7, was sequenced and analyzed. Atpk7 was demonstrated by PCR to contain an intron. The mRNA transcribed from Atpk7 was detected predominantly in root tissue by Northern blot analysis. The transcription start point was determined by primer extension. The deduced amino acid (aa) sequence of the putative product of Atpk7 resembles those of S6 kinases, cyclic nucleotide-dependent PKs and calcium-dependent PKs. From this comparison of aa sequences, the ATPK7 protein is considered to be a member of a novel subfamily of Ser/Thr PKs in plants.