Genes for subunits of respiratory chain complex I are found in mitochondrial, plastid and/or nuclear genomes with varying distributions in the diverse eukaryotic species. The intrinsic PSST subunit of complex I is a mitochondrially encoded protein in Paramecium but is specified by a nuclear gene in animals. In plants to date only the homologous plastid encoded NDH-K gene product has been described. The analogous plant mitochondrial protein is now identified as the 22 kDa complex I subunit and found to be encoded in the nuclear genome of Arabidopsis and potato. The cDNA sequences of clones isolated from both plants are 79% identical in the conserved coding region, while the 5' parts of the reading frames specifying the N-terminal presequences for mitochondrial import differ significantly. The expression of the genes examined in different organs of both plants by Northern blot analysis shows elevated steady-state mRNA levels in flowers. Hence, expression of the gene appears to be organ-specifically regulated by its transcription rate and/or mRNA stability. A 1.6 kb long genomic DNA sequence of Arabidopsis upstream of the transcribed gene region encoding the PSST subunit in Arabidopsis contains several putative promoter sequence motifs. The results are discussed with regard to the appearance of a nuclearly integrated, former mitochondrial gene.